XDM is now in 45. But I agree with you, I won't be without a 1911. I have two. An old Springfield that I have shot thousands of rounds through and my carry gun a DW CBOB. Love them both.I'm not telling you you have to have a 1911, but you should at least shoot one a little before making your decision. I won;t ever be without one (or three...) ever again.
As far as the XD is concerned, unless something has changed, the XDm is not available in .45. The m had some different grip patterns and maybe a higher capacity by one round, but again, is only available in 9mm and .40, I believe.
If you want a polymer (plastic) gun, make sure you also check out the S&W M&P series, I like them a lot.
Outside of all of that, got to a shop and fondle some guns, see what feels right for you as far as grip, balance etc...
Is this your first handgun?