Since both I and my son I bought our gear used, we started out with differently sized tanks: I got one tank of the classic European standard size, a 15L 200bar (some 100 cu.ft) and one 15L 300bar (about 150 cu.ft), while my son got the new standard rec tank around here, a 10L 300bar (also some 100 cu.ft). As my SAC improved (it's now somewhere in the range of 15-20 SLM (0.5-0.7 CuF/min) in a DS), I got a pretty nice correlation between my min gas time at depth and my NDL on air, so there was no need for the capacity of the 15x300. Since the 15x300 was just too heavy for comfort, I was pretty happy when I was able to swap it for a 10x300.
As time went by, I bought a couple more tanks (we now have a total of five tanks for two people), and since the 10L300 is more negative than the 200 bar 15 liters, I've standardized on 10 liter tanks even if I get about 10% less gas with that tank than I do with the 15x200 (even though the nominal capacity is the same). I need some 3-4kg less lead if I'm using a 10x300 than if I'm using a 15x200, and that's valuable to me. The 15x200 that I haven't sold is kept as a backup tank, since the resale value is pretty low.
IME, drysuit diving requires more gas than wetsuit diving, and besides, Al tanks aren't very common in my neck of the woods. So the "standard" European tank, a steel tank with ~3000 nominal surface liters (~100 cu.ft) fits me and my diving pretty well. If I'd be using a (US standard) Al80, I'd probably be gas limited on most of my dives. Perhaps even on nitrox. Since I mostly dive nitrox these days, I might have had use for the 15x300, but OTOH my buddy wouldn't have that amount of gas. Besides, being an old geezer I'm quite comfortable with having a bit of a margin WRT to my NDLs. Besides, in a DS any weight you can shave off your tank you have to add to your belt...
So, the lineup for my family is:
4x 10L 300 bar
1x 15L 200 bar
All singles. No ponys, I don't see any need for those. And no doubles; although quite a few of my clubmates like rec doubles, I prefer singles.