There's another consideration.  How much fun is it?  The Domino is fun to use.  Plus it's forgiving.  Dowels are the opposite.  They are not forgiving.  The Domino is my favorite joinery method for MFC and for plywood. 

Get the cheapest clamps possible that are made from metal.  Assembly is very very fast with Dominos, especially for MFC.  Pop the case together.  Have three or four clamps set up on the ground (or better a low height assembly table)  Set the case on those clamps place 4 clamps on top then tighten it all down.  Don't need much pressure.  Since you'll only be gluing in the mortises it's not messy.  Wipe off the squeeze out and while that case sets up start on the next.  You can have all 18 cases assembled in one long day. 

Also I'd consider the XL.  You can use if for everything the 500 does plus more.  Get the Seneca adapter set and you can use 5mm tenons for sheet goods.  And you'll have that marvelous 70mm depth for everything else.  Beds, passage doors, big tables and so on.  You can use your XL as the starter for doing big through tenon joinery.  Just square off the ends. 

You'll have it for the rest of your life and it will open up possibilities.  How about an fancy garden gate?  How about helping a friend with his cabinets?  How about doing some cabinetry for your church or for your favorite charitable organization?

I use a lot of pocket holes.  I have one of the older Kreg Foreman pocket hole machines and all sorts of pocket hole specific clamps.  I do it a lot.  A pocket hole jig is something every body should have.  For me it's indispensable.  But it's not as cool as fun as a Domino and it's ugly.  So I use Dominoes whenever I can.

Plus the Domino is clean.  Sawdust is practically nonexistent.

just to be clear for those claiming the dowels have to be placed perfectly is easily worked around.  If I need a little wiggle room I just make two plunges one on either side of the layout mark. the second plunge only takes an extra second.  This works really well as most of the time you are only off by .010"-.015 unless you mismarked. The 500 is a great tool also but the ddf40 is better suited for cabs. Also mafell offers slightly oversized bits but I prefer the double plunge.

Never handled a DDF40, but when do you plunge twice? After the dry fit right in which adjustments are found to be necessary? And that means you have to identify and mark the holes concerned, if there's more than one and go back to the drilling process for those holes and boards, a step not usually required with the DF500 when the wide mortises are cut.

I don't know which system is better for cabinetmaking, but on the aspect of doing (plunging) it right the first time, one can't deny that the DF500's tight-loose setting really gets it.

Whether or not and "when" double plunge will depend on several things
1.The the individual = doing the layout and plunging (how accurate that person is) the more accurate you are here the less double plunge will be needed.
2. The method of indexing = (jig or pencil mark) If using a jig that will locate the DDF accurately then NO double plunge is typically needed.
3. How accurate (flush) the ends need to be. = This tends to be the biggest driver for me on the double plunge.  Typically even if the dowels are only off a little you can still mate the pieces. However the ends being perfectly flush will be where the joint suffers "slightly" This also depends on how OCD a person is and the piece. Sometimes a piece is trimmed or sanded in after so its a non issue.  Also when I say "slightly" I'm talking about .008"ish.  I just did a piece yesterday that was about 32" long and had 8 dowels (4pairs) laid out by hand with a pencil and the ends were off by maybe .006" just enough to slightly catch a finger nail.  The parts were cut on CNC and laminated so no sanding or final trim was planned and they were exact same length.  If you are in a situation that requires "perfectly" flush ends then double plunge probably from the start.
4. Joint complexity = The more dowels and/or corners you have coming together the more likely you will benefit from double plunging. After awhile you develop a little voice in the back of your head that tells you when to double plunge. If your worried I would say double plunge from the get go it only takes a couple extra seconds the 2nd plunge is taking off so little you hardly even notice. I would do one side of the joint with single plunge (on the mark) and double plunge the the other piece (place cursor on either side of the pencil mark.) 

There is no doubt that the Domino provides a wider margin of error and is a great machine. It was actually my gateway tool into the Festool brand and still own it.  However for me personally I find the middle setting to sloppy for my taste.  I am typically pretty darn close on the layout and dont require such a large margin so I use the tight setting anyways. This is also my biggest wish for the domino is to have a "variable" Instead of perfect/sloppy and supper sloppy.  Does anyone even use the super sloppy ?

For me its like picking the favorite kid, and neither are perfect.  Here are my biggest likes/dislikes for each

consumable cost= there is no denying that dominos are considerably more than a dowel typically 4-5x this adds up over time
 
Mafell has better fence both in adjustability and locking (I wish the domino also had a micro adjust height control for fence)
 
The Mafell can drill "all" the system holes with one tool. No LR32 is needed.  (However I have a lot of love for the LR32 too)

The Mafell has a lot larger range up to 12mm and since its a round hole the 40mm depth is irreverent I have used the DDF40 to do the locating and drill the pilot hole and for heavy duty projects I simply run a longer drill bit down the hole and use dowel rod custom cut. easily getting 12" of length if needed.  So im my opinion it bridges the gap between the 500 and 700 in one machine better.

Plunging Ill give the edge to the Domino but its close, really close.  The DDF is harder to plunge on the first plunge but you do develop a technique the second plunge if needed is like plunging into butter.  The domino requires less effort or force but slower feed rate.

Exposed fastners hands down the Domino wins here and truthfully the only reason I hang onto the Domino.  I can do everything with the DDF that I can do with the Domino just as fast EXCEPT have an exposed domino.  I will never do another dovetail drawer The Domino is a speed demon for making exposed Domino drawers.  Drawers with no dovetails or no visible joinery looks cheap to me.  However so does a exposed dowel IMO but a exposed Dominos look really really nice IMO.  Its a toss up for me which I like better for best looking drawer box but the combination of speed and looks the Exposed domino drawer wins over a dovetail drawer every time. Even though I have never scientifically tested the strength of either I wouldnt be surprised if the exposed domino wasnt stronger.  However at that point its debatable strongest vs. strong enough as the wood will likely fail before the joint.   

Thats an interesting tidbit I didnt know the 700 had 3mm vs 6mm middle setting.  The 3mm seems much more reasonable. Actually I didnt know the actual specs of either I just knew I cut one on the middle setting and it seemed huge to me. Not sure what the super sloppy setting is but if I needed something bigger than the middle for regular joining I think it would be time to sell all the tools.   

Yea, I did mention the oversize bits in one of my other posts but a lot of folks just skim right over that.  The issues with the oversize bits are several incase you dont own any.
1. They are pricy
2. they dont provide much wiggle room I need to tap in 8mm dowels into 8mm holes but its humid where I am so that could be partly to blame.  The oversize bits dont do a whole lot the dowels are still snug but go in/out with fingers.  It still doesnt do much for wiggle room.  I also only tried the over sized bits on one half of the joint since the face drilling is done on CNC Perhaps if both sides were drilled with oversized bit it would make more of a difference.
3. The oversize bits increase the entire hole size which could make the one edge misalign as well.  The double plunge method maintains a consistent distance between the fence reference edge and dowel.

I had the LR32 prior to the DDF and got really good and fast results with it so I never bothered with the DDF but can see how having just one machine for all would be a big benefit for some.

As far as renting Festools here in the USA thats a no go.  I have never seen anyone renting Festool.   

 

Renting is for commerce as loaning for family and friends. I'll be willing to lend my PRO5 LTD sander (because I have two of it), but not my DF or Kapex.

I've seen people using those machines on the web without proper care, such as letting the Domino body spring back after a cut rather than releasing it gradually. Or letting the saw head spring back up. Many miter saw users are seen to be engaging in kickback prone operations, and I have no control of how my saw is used once it leaves my shop.

More than a few times, I cut workpieces for my neighbors on the SawStop, but again, I prefer them (including  woodworkers) not to do the cuts themselves because it has taken me efforts to tune up the saw to cut with such precision.