Round head claims to have more holding power but clipped head holds more
nails per strip which is important to someone who is making his living with
the gun. Clipped head nails are usually cheaper which is also important.
IMO I would look instead for which gun feels good to you as far as weight
and handling. Also consider the availability of service and parts. Price
tag is also something to consider. If you are just going to be a casual
user just about any gun will be adequate.

Both nails are going to have sufficient holding power for most applications.
As far as installing a rail to a deck I would say that a screw would work
best. Especially if your fastener is going into end grain. Also, a normal
framing nail will corrode away before the wood needs to be replaced.

If you just want a new toy, buy a self feeding screw gun for this
application. If you just want a framing nailer, buy it but still use screws
(stainless or non-corrosive coated) for your deck railing.