I own and like both! However, if you are planning on ever modifying your handgun, or buying more mags then I'd recommend the glock. Parts and mags are cheap and Available all over for the glock! Xdm mags are expensive and modifying them are expensive and difficult.
I always assumed it was to give the polymer guns a place to play. But then they upped the weight to the point that CZs are legal, so I don't really see the point. To me production weight +4 oz seems to make the most sense.