Which Domino to get will depend on what you want to do initially, and what you may anticipate you'll want to do once the door to joinery has opened up for you. Either tool is absolutely fantastic, and in a class of it's own!
If doing mainly panels, tables, cupboard doors, etc, the 500 will be more than adequate. If you're wanting to do large joinery like solid timber doors 35-45mm thick, the XL700 would be the better choice.
Personally I think if you had to buy one, the XL700 would be the better choice, as you have the largest options, plus the 8mm cutter will generally do almost anything smaller you'd need. If you needed to go smaller still for some projects, you could buy the Seneca adaptor that will allow you to use the 500's cutters.
This is another thread asking the same:
https://www.festoolownersgroup.com/festool-tool-reviews/domino-500-vs-700/They're not a trivial purchase, so these videos might be handy:
<<Edit: Inserted long YouTube Links - P.Halle, Moderator>>