My thoughts on the subject are that a polymer gun can benefit from being more rust resistant along with the lighter weight as above mentioned. Other then that I guess it just comes down to personal preference really on what you like!
For me the decision was made by me handling several makes and models, finding the M&P the most comfortable to hold, aim and shoot. I have only had one steel pistol other than my revolver, a S&W 469. It was nice but nothing points and shoots as nice as my M&P.
I would think it's the weight for carry..but if you do your homework on holsters and egronomics everthing will fall into place and you'll carry either...
It's like asking boxers or briefs. It all depends on what feels best to you. I want to like plastic guns, but have found that I can't hit squat with a Glock. Everybody is built different and some things work great for some and not so much for others.
I've never tried an XD but was able to shoot a Taurus 24/7 pretty well. I'm most comfortable with a metal gun and am a fan of aluminum frames right now. Someday I'll probably end up with a plastic gun in the collection and maybe I'll see the light then. If that happens, great, if not, also great. I'm satisfied right now with a commader sized 1911 with an aluminum frame. It generally puts holes where I want holes to be put, so that's what works for me.