i own an XD .45ACP Service, my brother in law owns an M&P .45..
the M&P is very comfortable, the feel is really nice.. it just sits in your hand very well.. but.. the trigger is horrible.. cleaned, lubricated, it feels like there is sand in it.. very gritty.. it does have a distinct break.. it has a somewhat heavy pull (it may not actually be very heavy, but it just seems that way to me)..
i don't like the take down lever to strip it.. nor, the mag disconnect.. nor the 10 round capacity mags.. the higher capacity mags are horrible looking, and a really bad after thought..
i do like the finish, and the slide serrations.. the adjustable backstrap.. and the grip's slightly rubbery feel.. it's a very positive grip.. the sights are nice on it..
as for the XD45..
it has a better trigger.. regardless of the long travel and reset.. it's smoother, and only gets more so after use.. it's easier to strip than the M&P (considering the take down lever).. 13+1.. the sights are very rudimentary.. they work.. i feel the barrel of the XD is of a much higher quality than the M&P..
the cons?? the grip is not as comfortable as the M&P.. it's a problem for people with smaller hands.. the slide release is easy to touch, causing the slide to go forward on the last round, or to catch in the middle of firing.. most people have a hard time at first getting used to the trigger.. usually pulling shots low and to the left.. there's an issue of when you reinstall the recoil spring rod, that if it's not dead center that you can lock your slide..
i know about the cons more on the XD than the M&P, due to owning one..
the XD is so accurate that you end up forgiving the short comings of it..
Springfield has the best customer service and they warranty the XDs without question..
yes, there is a lack of parts for the XD, but SA has recently allowed parts for sale.. so, it's not that hard to get parts.. i'm not sure about parts availability for the M&P.. i would figure it's about the same..
i prefer the XD.. because it's the one i have.. the M&P feels good.. but, in shooting my BILs M&P, i couldn't hit anything with it.. that's just me, and i'm sure that others have much better experiences with the M&P than i have..