You will get a bit more power - but not 18/15 = 20% more.
But you will stress the batteries less - 10 cells in pack versus 8 cells in 15V pack means 20% higher current load on the 15V pack cells. So longer life as well as more cycles out of the 18 packs.
If you are a heavy user, the 6.2 Bluetooth packs may be the best approach.
With higher capacity you buy not only individual runtime +20% or so, but ALSO the battery longevity is increased by more than 20%, think 25% or so. This is both a result of less frequent battery changes/cycles as well the 3.1 Ah cells being less loaded relatively ref amperage-to-capacity compared to the 2.6Ah cells in the 5.2 bats.
Overall, these aspects combined, you are likely to see anywhere from 40-60% better runtime and 60%-80% longer batteries lifespan with 6.2/18V versus a 5.2/15V packs.
On the durability, another thing that helps is to charge more often. It is better to charge the packs already when 50% empty instead of waiting for the pack to blink telling you it is spent too much. This can add a LOT of life to a pack.
A pack which will last 1000 cycles when cycled 95%-30% will easily last 2000+ cycles when cycled 95%-50% most of the time.
And even better is not to charge the bats 100% either - for most Li-Ion cells the most cycles one gets with something like 85%-60% cycles. That way the pack can give often in the range of 10,000+ cycles. The Festool chargers are smart, and never go 100% in the traditional sense. But it still means that pulling a pack off the charger when it shows <5 mins is actually better than leaving it there. For that pack.
Also, the battery tech improved in last 5 years a lot. The cells in new batteries are better a bit too on average.
With my TSC (use 5.2 18V packs) when on a bigger project, I would swap the batteries once I start feeling they are getting weaker - this is usually around the time they drop to 2 leds. Having two SCA8 charges, I will not manage to drain the replacement packs while the first set gets recharged enough.
