I have a full size XD9 (actually an HS2000, but who's counting?) and a full size 1911 (Springfield Armory GI.45). I've carried both, but I've never carried them IWB. I carry them OWB. My IWB guns are a S&W 637 (most comfortable gun I've ever carried) and a Bersa Thunder 380 (not a bad carry, but it doesn't melt away and disappear the way the S&W does).
Just looking at them side by side... I wouldn't want to carry either the 1911 or the XD IWB. I'm fairly thin, and neither one would conceal very well or be very comfortable. If I had to pick one for IWB carry, it would be the 1911.
Also... whether it conceals better or not, a single-stack full size 1911 must have a smaller overall grip circumference (the measurement you'd take if you wrapped a string around the grip). I say that, because my girlfriend has small hands with short fingers, and she can't comfortably shoot most of my full size pistols. She can handle the 1911, though. Then again, that may not all be in the grip. It might also have something to do with the shorter reach to the trigger on a 1911.