I'm not a technician so I cannot give alot of input - in general the difference is 'payload'. I for one pretty much only use 2 3/4", sometimes 3". I have never bothered with the 3 1/2's. I do not see the need for MY style of shooting and certainly the cost is inappropriate. Impact for steel loads, and correct me if I am wrong guys, is basically speed, since steel is so much less dense than lead. Therefore, I go for fast loads. The concept of the 3" and 3 1/2" is to offer more shot with that same level of high velocity.
Help me out everyone!