To the OP, my recommendation is to shoot both and see what you like. If you want to have responsible fun, then I would recommend the .40 purely for the cheaper ammo. However, the .45 does have more stopping power for protection situations. When I bought my XDm, I was torn between the 9mm, .40, and .45. I ended up with the .40 as a compromise between the two ends of the spectrum. Once I shot the .40, I knew it was enough for what I was looking for.