I know a guy who has run a p90 for the last 6 years outdoors. It's essentially a trimmed down LMG, runs like a beast. Dude mows down tons of guys, and carries 10 mags on him in drop leg and crossdraw holsters. 500 rounds at realcap, one mag = 1 or more kills. They shoot straight, are reliable, accurate and are sturdy.
Personally, I can't stand 'em. I don't like the way they feel, I can't really shoulder them or aim them especially well. But to those who like 'em, they LOVE 'em. Are you one of that crowd? While cross-mag compatibility is awesome, it's not end-all be-all.
1911 you can't go wrong with, ergonomically. Points naturally, and if it's a decent gun, will hit what you point it at.
In the end, it's up to you. P90 has a long list of pros, but that's if you enjoy the way it feels in your hands. The cons are obvious, and already listed. Try one out if you can, see if it'll fit you. Then it's a matter of finding ways to store the mags, which can take some doing, though the fact that they are more popular these days helps.
- Fides et Amicitia