Apart from the nail head: the nail material matters here, too. What wood species and what finish are you going to have on the fence?
If it is an unpainted cedar, you will likely want to spring for a stainless steel nail (if nailing) or a coated screw (if screwing). In fact, even with those fasteners, you will want to shop around. This is where you probably are going to want to spend a little more money on a higher quality fastener.
Uncoated or galvanized nalls/screws will react with cedar (and some other species) and leave black streaky marks.
Now... if you're painting it... it may not matter as much.