I've always been partial to the 9mm simply because I find it easier to control. I can get 2 rounds on target faster and more reliably with 9mm ... and just can't seem to make that happen with .45. I guess the question to ask is what is the max caliber you can effectively handle? If the answer is .45 then go with the larger caliber with the better stoping power. I'd go with the .45 if I could manage it better.
I carry a Service Model XD .45; it requires me to wear a coat or an untucked button-up shirt. I can fit it SOB, but that's akward (and unsafe). It hides under a regular shirt (I have a supertuck) but it definitely makes you look lopsided. I say it's not hidable that way.