I have an M&P 9c, and have put a few hundred rounds through a fullsize, my brother has a P99 A/S, we shoot together often, and swap pistols. Both are completely reliable, and comfortable, the P99 is slimmer, smaller than the M&P fullsize, but bigger than the M&P compact, and is quite accurate, can be decocked for what feels like a DA/SA operation, and the trigger is fairly light, but a little spongy, and feels like it releases too far back, and travel is too long, in A/S mode it is the same light pull, but twice as long as normal. I like the P99 a lot, it is a very comfortable and quality pistol, and has the euro trigger guard style mag release (love it or hate it), but the trigger feels a little too light and sloppy, the sights aren't as good, and while it feels more rigid than an XD or glock, it has noticeably more flex than the M&P.
The M&P on the other hand has a crisp trigger, and is fairly light in the 9/357sig/40 pistols (5lbs), the 45 is still crisp, but has a heavier pull (8.5lbs). The M&P only has a hinged type trigger safety, but has a traditional button type mag release. They have a stainless steel chassis molded in the frame, and have about the least ammount of muzzle flip and recoil I have felt in a 9mm pistol that wasn't comped. My M&P did seem to require a slight break in period of about 250rds, where it would not go completely into battery about 3 or 4 times in that period, but has been flawless in the 1000rds since, the P99 was utterly reliable out of the box, and so far hasn't had a malfunction of any type. Both are very accurate, but the M&P is easier to accurately rapid fire, and I can swap mags faster in the M&P, and the novak type sights are better on the M&P. As a bonus, S&W has a coupon on their website for a $50 rebate, and 2 free mags untill january or so. Either pistol will serve you well, as both are really good, for CCW the P99 is a little slimmer and more comfortable than the M&P fullsize, but the compact is easier to conceal, as far as shooting, the M&P is easier to fire more shots accurately from the draw, and all around a simpler and more user friendly design, and if I had to choose between the two , the M&P would be my choice, full or compact as need dictates.