It may depend on which grip size suits you best. The XDm 4.5 in .45 ACP is my choice for a couple of reasons. I have larger hands, so the .45 size grip is a better fit in my hand. I handload .45 ACP and one reason I got away from .40s after trying nearly every pistol under the sun, is because the .45 will easily cover all the same bases and do it at much lower pressure. Max. standard pressure for the .40 S&W is 35,000 PSI. Max. standard pressure for the .45 ACP is 21,000 PSI. One reason why .45 recoil is more of a firm push than being snappy.
And as johnnyblack mentioned, there's that hole in the target thingy. It's hard to beat what .45 SWCs will do to a target, especially when you're fortunate enough to put them all together in one cloverleaf. If you plan to shoot .45 ACP SWCs, you'll definitely want to get the XDm as it's been modified to function with that bullet whereas the standard XD is not.