One other thing I forgot to mention... Cycle Length.
Besides the size, weight and girth differences between 40's and 45's there is also the distance the slide travels - the longer it travels the less felt recoil as the spring has more time and opportunity to soak it up slowly.
The cycle length of a 45 is longer than a 40 by a few millimeters.
The cases are only about 1mm longer, but the overall length with a round seated is a few more (it has to be able to cycle out a complete round too, not just the case).
Point is.. the shorter the cycle length the "snappier" it will feel with all other things being somewhat equal (40 vs 45 energy levels).