Need more information - are you shooting trap only, or all clay-bird games?
If you're shooting trap, a semi might send spent hulls and the guy to your right spinning, so a pump could be better.
If you're shooting skeet - either can work very well. I shoot a semi, pump, and O/U for skeet. Sometimes, a semi can fail to feed (otherwise jam - even clean guns have done it). If that's going to bother you, you might want to consider another option. Same for sporting clays.
If you're hunting with it, you'll have to decide what you want to hunt with too.
Try to shoot both types before you make a decision.
Good luck - SD