This is becoming a common question on this NG.
I have a 15 gauge, a 16 gauge and an 18 gauge nailer. (Home Use, only)
My suggestion is to get the 15 gauge and plan on getting the 18 gauge,
later. It's not the 16 gauge won't work, it's just that that for specific
applications, the 15 gauge works just a bit better, while, at other times,
the 18 gauge is preferred.
Your mileage may vary.
You might as well plan on a 1/4" crown stapler, too. In fact, for someone
just starting out, I'd recommend a 15 guage finish nailer and the crown