Definitely 9mm over .40 for all the reasons already discussed in this thread, but I'm a big believer in having options. I remember not too many years ago when 9mm was wildly overpriced or out of stock everywhere, I bought several cases of .40 and it's still on a shelf in the basement. Did move on from some guns though; kept a stainless slide USP compact and a homebrew Glock. A few years ago I was bitten by the poly80 bug pretty badly, I ended up building a few, and one I built into G23 type with a bunch of aftermarket parts. Shot it enough to break it in and make it cycle reliably, and I think it's been in the safe ever since, but I like the idea that if I ever have to relocate professionally to a left coast state, the USP is on the roster (IIRC), and I can have the frankengun just for range play.