Depends on whether you want a finish head, clipped head, or full head. For siding, you should use a nail with a head on it. Clipped, or Round, does not matter. For the deck, I would only use screws, no nails ever. Sounds as if you are looking for a framing nailer, most of which will accept smaller nails. If you ever want a nailer for finish work, consider an angled finish gun. You're right, so many to choose from, and they all depend on what your specific need is.
I own the following,
Pasload framing nailer
Hitachi angled finish nailer 15 ga
Porter Cable angled finish nailer 15 ga
Senco angled finish nailer 15ga
Bostich 2" Brad nailer 18 ga
Porter Cable 1 1/4" Brad Nailer 18 ga
Senco 1 1/2" Brad Nailer 18ga
Senco 23ga micro pin gun
Porter Cable 23ga pin gun
Bostich 1 1/2" Crown Stapler
I prefer the Hitachi over the other 15ga and the Porter Cable 23ga over the Senco.