Well, it really depends on the intended use.
If this is for Range/IDPA/etc. use, you'll need a double.
If this is for carry, I'd go with a single, unless you are in a capacity restricted state, then I would go with a double.
It's all a matter of perspective.
I carry 14+1 with a 16 round backup.
If I could only have 10 round mags, I would carry two spares and end up with the same total rounds.
If I think it might be a good idea to carry a third standard capacity mag somewhere, maybe I need to bring a carbine.
