As far as a definitive answer, here goes
Primer + Paint - will even out the surface to produce ultra smooth paint jobs. However, it is more brittle than paint and WILL flake if given the chance.
Paint only - Finish won't be as good as Primer + paint, but it will still look good as long as you provide enough coats and do it right. Paint is more flexible than primer so it won't flake as easily...it still can and will flake though.
Best bets for paint are those that are plastic based...like latex. 2 part paints (like car paint that requires a hardener) are much too hard and won't flex much at all, these will flake the second you hit anything!