A screw is the better fastener but as Coley said, not everyone likes the round plugs and they can stick out like a sore thumb. I personally like to use square, proud and chamfered pegs on stuff that needs to look good which needs to take a solid fixing. You would only need two screws per board on each fixing point about 20mm in on each edge of the board for a good solid fixing which will prevent the boards twisting. Also as Coley said, gives a good opportunity to use stainless steel fixings which while can be a pain to put into Iroko because it's so hard and tends to snap screws (Give a decent pilot hole first) it does mean you won't get any rust seepage down your boards in years to come.
Nails, especially the wire nails and brads tend to get pulled out by the timber over time if there isn't any glue or other bonding holding the timber down as well. For exterior stuff like gates, I really do prefer the old cut nails or roseheads as they look nicer and traditional as well as far great holding power in timber than wire nails. They are also much better at holding timber together if clinched over on the back and fed back into the work, you see many old ledge & brace doors done this way.
