For suppressed use I wouldn't use either a 10mm or a .40 cal. Now I do think the Glock 20 is a really cool gun, but it just doesn't scream suppressor to me.
Sigs are supposed to be very good for suppression, with the edge going to the P226 over the rest of them (probably the extra bit of barrel, but I'm not sure).
If you want to go through all the trouble to download and trouble-shoot your own subsonic 10mm, feel free. But for a first suppressed pistol, I'd stick with something in 9mm where you can find factory loaded subsonic ammo at a pretty decent price in comparison to other 9mm ammo types.
My plan for a first suppressed pistol is going to be either a Sig 226 or 228 in 9mm (already have the 228) with a Ti-Rant or Osprey. Though I may also pick up a TM Poseidon just so I can have one of those cool little micro suppressors to play around with on occasion