I could go on for pages about this, you'll always have different fans of different platforms out there and each have thier pro/cons. I am a 1911 guy, but I own everything and anything in between as far as platforms'syles are concerned. This is a huge debate among allot of folks. But it all depends on what YOU are comfortable shooting and which pistol YOU can handle and shoot best, period. As for the caliber...well, 45 punches a nice hole, but would you rather have 7rds of .45 in a single stack or 15rds in a 9mm like a G17? 9mm might expand but a .45 will never shrink some say, but theres plenty of great performing ammo out there today that would suite a defense weapon just fine in 9mm. Plus, if you want to be good with this pistol PRACTICE PRACTICE PRACTICE!!! And 9mm is a ton cheaper to shoot than .45, so, you'll get more range time. Range time=better performance. I'd rather shoot great with a 9mm that I practiced with allot than so-so with a .45 I didn't shoot to much. Or, if cost isn't an option, get a good ole slab-side 1911 and shoot the bee-jeezus out of it!!! Plus 1911's can't be beat for customizing, the options are endless.