There is an ever so slight difference in the grip contour of the P225 that results in a noticably better feel. The P228's double stack mag results in a more squared off frame. While the dimensions are pretty much the same between the two, the P225 is able to "flow" better. It's the difference between "excellent" (P228) and "perfect" (P225). Gun wise they are the same, one just hold more bullets. Gun-in-hand wise the P225 is just a better feeling weapon.
Ross