Choose the nail size you want to use first and that will dictate which nailer you end up using. Generally, the following is true:
15ga finish nails use a nailer with an angled magazine and come in either DA or FN (Bostitch only?) versions. DA have a round head, FN have a "t" head
16ga and 18ga finish nails use nailers with a straight magazines
I would use 15ga for exterior trim and/or large interior trim,
and a mixture of 16ga and 18ga for interior door or baseboard trim. For what you are describing, I would probably use either a 16 or 18ga nail. May want to go look at some nails first.