I have a bunch of nailers. I use the 15 gauge when I'm working on weightier projects such as door jambs, 8" baseboard, etc.
Most of the time a 16 ga. is preferrable. I really like my Paslode impule angled nailer. Aside from the fact you don't need to lug a compressor around, it can fire 1 1/4" nails which are nice for small work. Whichever you buy I highly recommend an angled nailer because it's easier to reach into tight places.