I keep hearing/reading the double vs. single stack debate.
why is one preferred over the other?
For myself, the question comes down to (a) feed reliability and (b) capacity. So long as a given gun is exceedingly reliable, then it doesn't matter whether it's single- or double-stack, to me. But, I am partial to having a higher number of rounds, all things considered. Which, I suppose, predisposes me to select guns with dual-stack magazines.
I'd prefer 10+ rds. I'd prefer a somewhat smaller grip circumference, though some larger ones fit me fine (ie, CZ's P-01). Browning made the BDM 9mm, which was exceedingly thin for a steel-framed, dual-stack gun. It held 15+1 and was thinner than a 1911, due to the design of the frame, integration of the grip panels inside the frame instead of mounting on top. The Browning Hi-Power isn't bad, either. Though it's noticeably thicker than the BDM, it is decidedly smaller than most every other dual-stack gun out there, so far as my grip is concerned.
Nicely, there are a couple of choices on the market. The HK P2000SK, for one, has a suitably small grip circumference and yet uses a magazines of 10rds in the 9mm flavor. A great combination: 10+ rds, dual-stack to get the round count, yet small enough to be considered a sub-compact. Could be a bit thinner, but 1.28" isn't bad.
On the other hand, shot placement is everything, why would you need more than 8 or so? If you are a good enough shot you shouldn't need as many rounds.
Chest pounding aside ...
Why? Because not everyone is a marksman. Nor, in such a fluid, stressful situation, would you want to rely on being one. Almost everyone isn't, actually. I would think it's obviously better to have a bit of cushion, particularly if from a size/space consideration it doesn't cost you anything. If there's no downside for a given person, it's hard to see how having more rounds could be a detriment. Can't hurt, and it might well mean the difference between life and death in a real situation. WFM. YMMV, as always.