Rounded 5/4" decking. On any cut edges, I make a new rounded edge to complete the look.  The decking is screwed down, 2 over each joist.

Occassionally I have to use 2x6 decking to match up an existing style. The reason I use 5/4" decking is that its much easier to get lumber where the proper side is facing up. If you look at the endgrain of most boards, you will see a circular pattern of grain. Ideally the board should be oriented in such a way that these circular (annular) rings arch toward the sky. The board will drain water faster and better this way, making the board and finish last longer.