For a list of the different brands of nail guns and reviews click Nail Guns
The only finish nail guns I use in finish carpentry come in two sizes. My big finish gun is a 16 gauge. The small finish gun or brad nailer is an 18 gauge. Hence the bigger the number the smaller the gun.
I also have a 15 gauge but don't use it to much any more for trimming as it leaves a bigger hole. These guns are pnuematic and do require an air compressor with hoses.
I have tried the airless models which have a battery and or a fuel cell in some models but found them to be to slow. They seemed to work ok but when in business, time is money and I was familiar with the performance and durability of the pnuematic ones.
16 Gauge Finish Nail Guns
I have not tried all the models in this size available. I did some research on the guns I have and it paid off for me. The manufacture of the 16 gauge I use is Hitachi. The gun is a little expensive but the quality and durability is worth it. The guns get hard constant use and are basically bullet proof.
The nails I use in this Hitachi gun are 2 1/2 inches. I refuse to buy all kinds of different sizes to carry with so I opt to trim with a little longer nails. A lot of the time you don't need the length but in certain situations you will be glad you have it.
A good cheaper model I have used is the 16 gauge Porter-Cable. This gun worked well but did wear out quicker and again this is with daily heavy use but on the other hand could possibly last a do it yourselfer for years. As always do a little research and read some reviews before ordering your gun.
18 Gauge Finish Nail Guns
In the 18 gauge size again I have just stuck with a certain manufacture which is Senco. And again because this gun stood up to the heavy usage. The only size nails I carry in this size gauge is 1 1/4 inches. In some models of the 18 gauge brad nailer you can shoot up to 2 inch nails and in different situations this may be nice because of the little hole it leaves.
Again the Porter-Cable in this size gun worked well and the Porter-Cable I had did shoot the 2 inch brads. I also had a Paslode in this size that worked ok but both guns got phased out and replaced with the Senco when they wore out. I want to stress again to do your homework and read the reviews before ordering your nail guns.