I have two primary calibers. I've selected the 9mm, and 223. I tend to buy 9mm's as my primary side arm and 223 as my primary assault rifles. I chose only two calibers so that I can stockpile ammunition without making it so complex. I have XD's, M11s, and several other in this caliber.
friendly piece of advice, stock at least one other set of calibers for each pistol and rifle even if it's not your goto. Diversity is good. Especially if you already have quantity in your calibers ofchoice.
I fire in 9mm and 223 as well but in the last year I've also been buying 45 and 308 just to have it because a lot of other people do.
$.02
Glock 17 is a nice piece, had it for a few years. Like someone else said though I don't like glocks a lot but you can't beat their reliability.