Just a heads up, S&W is no longer making the M&P in .357 Sig; you would have to either get a used one or get a .40 and then a conversion barrel.
ETA: I take that statement back, I just checked S&W website and they do have them listed again, but they are a little more expensive.
However, I would still much prefer that to any XD. The XD just feels much more 'plasticy' to me and the depression for the grip safety makes it much less ergonomic (again, for me).
If they don't have equal mag capacities, I would expect the M&P to have a higher one. The XD service (in 9mm) has a 16 round capacity, while the standard M&P9 full size has a 17 round capacity.
I also went back and checked mag capacities and the M&P is a 15+1 while the XD is only 13+1...