H10,
Its funny you ask that. My dept just switched from the Walther P99's to the MP. Honestly, I guess it is a personal preference, but the MP was BY FAR AND ABOVE AND AWAY the winner and preferred choice.
The Walther's were OK, but the MP was favored by 99% of our 300 man dept. For me personally, the MP is a lot better gun (feeling, shooting, grips, trigger, etc, you name it, basically everything about the MP) in my opinion.
I personally would pick up the MP by choice over about anything else (except my CZ's), but the Walther would be way down the line.
I cant even compare these two together, again, my opinion.