Alf,
I like the P99 a lot. I even use one in a couple segments of The Best Defense, but you're going to find the XD a more efficient gun to use in a defensive context. The location of the decocker on the P99 is a bit awkward for most people (even after practice) and, of course, the complete lack of one on the XD means consistent trigger weight in all situations and the lack of an extra step after a string of fire.
I'm not getting rid of my P99 anytime soon, but I'm also not worried about extra mags as it is purely a pleasure gun/novelty for the collection.
-RJP