ok ,bowtex ,now that everyone has touted "their " particular shotgun ,let get down to the nitty gritty.
In order to enjoy what ever shotgun you chose it must fit and feel good in your hands. This is the most important thing to consider when looking. You said the Benelli is within your price range so as that being the top end look at every other model then decide. Benelli and Beretta are probably the top two auto out there at the moment. The Beretta's have a nice feature as one is able to change cast and drop some on them with a shim that come with it. There's nothing wrong with Browning,Franchi or Remington (although the Remington might need a bit of tweeking before it is reliable) So again get what fits and feels good to you, that's more important than price.
Now as for as chambering , if you want to get a shotgun chambered in 3 1/2 go for it. I'd imagine guns built to handle a 3 1/2 has to be a bit stronger than a 3 or 2 3/4 chamber. That's only a guess and a gut feeling, but beyond that if you go with a 3 1/2 chamber shoot 2 3/4 or 3 inch thru it if you feel the need for the 3 inch. The money saved on ammo would be better spent on istruction and time shooting clay birds. IMHO 12ga. 3 1/2" is right up there with porting. Both are unnessasary and an added exspense. You'll hear the tales of 50 + yard shots etc , but believe me people that can consistently bring down the bird they are shooting at with one or two shots are few and far between. Besides even in an auto a 3 1/2 is by no means a pleasure to shot. I'd rather sit at the shooting bench with my 375 H&H than a 3 1/2.