I don't believe in buying the same basic gun twice so I wouldn't buy, say, a G19 and a SA XD9 (others might, YMMV). That said, there is a gap in my carry gun options. Aside from my two "house guns" (Ruger Security Six and P85), here's what I currently have and carry...
STI Trojan SS 1911 (full size) - 45ACP
Sig P229 - 40S&W
Kahr K40 - 40S&W
Ruger SP101 (2.25 in) - 357 Win mag.
Given that I'd like to have a nice array of handguns as carry options, I'm considering two possible paths:
1. Get a nice 9mm - G19, H&K P30, or Walther PPQ are the options in the running, or...
2. Get a pocket handgun - S&W M&P 340 and Kahr PM9 are the options in the running.
I'm currently thinking that the 9's I mentioned basically "overlap" with my K40 as far as size/carryability goes (the benefit of a 9 being greater capacity, of course). So, right now, I'm favoring a pocket handgun, because (1) I think I'd carry it a lot (and not in substitution of one of my existing firearms - my minimal overlap rule) and (2) a pocket handgun fills a more obvious gap in my current carry rotation. A pocket handgun is also "more different" in that, unlike the other firearms I've mentioned, it doesn't need to be holstered, other than pocket holstered. BTW, I do realize that this is more a "want" matter than a "need" matter... but I know you'll understand - LOL.
My bottom line is that I think I'll go with the S&W M&P 340 as my next purchase. To me, this seems like a no regrets move. But I could be wrong... Opinions?