After owning and shooting the M&P45 (full size) and HK45C, I prefer the HK. It fits my hand a little better, and although the M&P is in all likelihood well engineered, the USP type design has many more successful miles on it. Recoil seems about the same for both pistols. I know that the standard M&P is about .5 inches longer than the HK45C, but otherwise dimensionally they're very similar.
For me it was worth the extra money. Good handgun fit has always been important to me.
"Life is short, but the years are long." - Robert A. Heinlein