The different sizes are:
First fix, full head - these fire 2.8 to 3.3mm 20° plasiic or wire collated nails. Not very common
First fix, clipped head - these fire 2.8 to 3.3mm 30 to 34° paper collated nails. The most commonly used type of first fix nailer
First fix nailers are also known as framing nailers (because they are used for studwork and roof framing)
15 gauge second fix - these generally fire angled round 15 ga nails from 32cto 63mm long. Used for heavier 2nd fix work such as skirtings, architraves, etc. Not common the UK
16 gauge second fix - these are made to fire either straight or angled flat strip 16 ga nails from 32 to 63mm long. Straight nails are also available as short as 16mm, although not all guns will fire them. Used for heavier 2nd fix work such as skirtings, architraves, etc. The most commonly used 2nd fix gun. Angled guns can be easier to get into tight corners but have a more limited range of nail sizes available (TBH nails shorter than 32mm are of limited use)
18 gauge second fix - these are made to fire straight flat strip 18 ga nails (also called brads). Used for lighter 2nd fix work where less obvious nail holes are required such as pre-finished skirtings, architraves, etc. as well as for glued assembly work.
Guns to fire smaller nails such as 23 ga finishing pins are available
Second fix guns are no good for framing work - the nails they fire are too short and too small
There are two main types of driving mechanisms, gas nailers (which use a small battery and gas canister to drive nails) and cordless (battery only, generally 18 volt) nailers
As a DIYer I doubt that gas will benefit you - gas canisters can leak, the gas has a limited shelf life and the nails are more expensive. The guns can also be more finicky to deal with if/when they go wrong (and all nailers go wrong from time to time). Against that gas nailers are generally a better bet when nailing hard, close grained hardwoods than cordless nailers
For your chosen task I'd really be looking at a 16 ga or possibly a 15 ga cordless gun. That means the deWalt DCN660 that you favour would be suitable. Having owned one (it is still my backup gun) I can attest that they are good for the cladding task you want to do. The flywheel mechanism needs to e kept clean but NEVER oiled as it is a friction drive. The only issue with that gun is that it doesn't perform well on dense hardwoods, as I've already stated
Sorry, there is a lot there, but it is a fairly broad subject
