Its a toss up:
The .45 has a greater stopping power yet has a very slow ROF and a smaller mag due to the weight of the bullet.
The 9mm on the other hand has slightly less stopping power yet it has a higher ROF, it weighs less, and its accuracy is outstanding.
But if I had to chose I would go with the FNP90, even though its not on the list, considering its a 5.7mm.
http://video.google.com/videoplay?do...&q=p90&pl=true
there is an awesome video of the P90 at work.