I'm not a fan of the P99 as a defensive pistol, as I believe we discussed elsewhere, unless it is the model without the decocker button, or unless you can hit the decocker button with your strongside thumb. Others will disagree.
The Glock/M&P thing comes down to ergonomics and reliability. The Glock has a proven track record in reliability. The M&P does not- yet. The M&P has more flexible ergonomics (multiple backstraps) and a trigger that most people would consider better.
It really comes down to which gun works better for you. Me? I'd choose the M&P. But if I was forced to choose the Glock, I would not feel inadequately armed.
Mike