If all you plan on doing is skeet and trap an O/U is your best bet. The extra weight if the second barrel will improve your swing and follow through. These are vital to these two events, as an added bonus the shells pop out into your hand.

If you plan on expanding your horizons into field hunting a light pump or semi-auto is worth considering. The extra weight that will help your swing will be the same weight that rips your shoulder off lugging a heavy gun around.