Many semi-autos are ammo sensitive. Go with a recoil operated if you go semi-auto. Then you can fill the mag tube with anything including mixed rounds without worrying about your gas tube settings. Also beware of the spray and pray philosophy. You may just start pulling the trigger again and again until maybe you run out at a critical time whereas with a pump there is a physical action required before each shot giving time to think or react to circumstances. For aimed shooting a semi may be better as you don't have to rack a round each time and possibly lose your sight picture. A disadvantage is that autos usually cost more than pumps. You can pick up 2 or 3 Mossberg 590's for the price of a Benelli at most gunshows.
As for myself I have both, several Mossbergs (one dedicated social gun) and a Benelli M121. I tend to favor the Benelli though. But I do practice with both. Actually I'm better at busting clay birds with my Benelli than with any of my other 6 or 8 shotguns that I own.
The Benelli is an excellent weapon. You may also want to examine the Beretta. It's a couple hundred cheaper than the Benelli. It's also the lightest auto I've handled.