I buy guns purely because I'm interested in that particular make/model. I'm saving up for a benelli m4, an Accuracy international .338 Lapua, a k98 mauser and somewhere down the line a HK mark 23. there are many guns I'm looking for, and when I have the cash I'll likely get what's available at the time and what interests me.
Capacity doesn't really play a factor into my purchasing decisions. if capacity is a problem, make plans for reloads.
The only time I've thouroughly planned a purchase was when buying my HD pistol. I liked the .45acp caliber, and liked the 1911 platform, but I did my research and ended up getting an HK45. 10 rounds of .45 is not inadequate, and should it prove to be, I have 3 more mags waiting their turn.
Imagine what I would do, if I could do all I can.