Doug, other than magazine capacity the P99's are very similar. I personally would get the P99 Anti-Stress Mode 9mm 15rd 4" Poly Grip if you are not in a capacity restricted State.

I would see if there was anywhere you could handle them and then get the one that "feels" best to you

It looks like there are just small differences other than capacity. Such as weight differences and dimension differences.Tthough both of those are very minor differences. So it honestly is up to personal preference. My advice is to go to a store or range if possible and hold the two in your hands to see how they feel.