If I could have the perfect setup it would include a full size and compact from the same manufacturer with the same configuration. One for home defense and one for carry. Some manufacturers are making the larger sized mags work in the smaller framed pistols with spacers or sleeves, HK, Glock, and S&W to name a few. If you were going to have one of each size, full and compact, would you buy different calibers and if so which one would you get in the full and which one in the compact?
I go back on forth on my thinking of this one, sometimes the larger frame with 9mm and a compact in .40 makes sense and at other times I could switch that up or even get the same caliber in both and then share the larger mags.