I would highly reccomend that you get a left hand gun. This way,you will be able to operate action from shoulder without empty shotshell casings flying across your face.
Now,as to 3vs 3 1/2, there is not as much difference between the two as a lot of people believe. With a pump action chambered for 3 1/2,you will be able to use ANY shotshell length on market. You have to ask yourself just how much are you really going to need supermag shells. The 3 1/2 shells are known to have more recoil than 3" or 2 3/4 shells.