Well presently I'm still using a 12 ga ,,,and shoot 2 3/4 inch turkey loads . With the advance of the loads made for 20 ga. and shot loads per shell being on the same as many 12 ,,2 3/4" ,,,,combined with a lighter gun , and many claiming excellent patterns out to 40 yards ,,,,I just might change over to the 20 .
I realize I have not addressed the ''advantages or disadvantages '' of the 3" or 3 1/2" , but will offer this point . I hunt the woods and strive to set up in closer quarters with the birds . Basically , the way I hunt , neither the 3 or 3.5 would improve at all over the 1 5/8 oz. , 12 ga. I use now . Plus my old neck bones tend to get out of wack from the heavy recoil of the longer , heavier loads .
However , my step-son uses a long chambered Mossberg 835 . With the present choke tube ( which I fail to recall exactly what it is ) his gun shoots about even up on patterns with the 3" or 3.5 ". He generally shoots the 3 " .
Oh yeah ,,,,WELCOME ,,,from Ohio