I'd take the GP-100, too. In a New York minute.
The P-90 is an excellent gun, but having handled several I've found I can't reach the safety with my thumb when holding it in a firing grip, even with the safety lever in the down, on-safe position. At least with my Beretta 92SB Compact I can flick the safety off without changing my grip...
Also, I have a 50th Anniversary Blackhawk in .357, so I've already got a fair assortment of ammo on hand to shoot in a GP-100. I was actually looking for a GP-100 or Smith 686 when I found the Blackhawk instead...and a GP or 686 is still on my short list of guns I really want. Preferably with a three inch barrel and (if I get a GP) a roundbutt grip.