I have been fortunate enough to win one of each in the last few years. The XD was a regular 4" version, not the tactical, and the M&P was also the standard length version. I was very impressed by both. They both worked reliably, were accurate, had good triggers for what they were, good ergonomics, etc.
The M&P felt slightly better to me in my hand. The bore is lower as Will mentioned and I prefered it's recoil impulse, but this largely personal preference too. I put them both up for sale because I didn't really need either, with the M&P $25 higher. The XD sold and I still have the M&P.
I don't think you would go wrong with either. If you like the XD you have and want to keep everything exactly the same, do like Gunonedown suggested and go with the XD Tactical. If you want to mix things up and try something different, the M&P is no slouch. Like Will mentioned, I would personally pick up the 9L or M&P Pro as they compare more equally with the tactical.
I put them on very even ground with a Glock, and that of course is saying something. The M&P feels better to me than the Glock, and I like the metal magazines better than the metal-lined plastic Glock mags. The plastic can get scratched up and drag some while the metal tend to drop free more easily. It's all trivial stuff though as they are all very good. It is amazing how much better the choices are now than they were not that long ago. My $.02 anyway.