Are you talking about 3" round vs 4" round or 3" round vs 4" oval? I am not sure that I have seen 4" round ones that often. If it is round vs oval, then both are OK. The ovals usually have an end cap, so I guess it really depends on what looks best to you.
I would go with the 4" maybe even the 5", I'm sure you get plenty of snow in PA and the bigger the bar, the less likely you are to slip on the way into/out of truck. I know here in MN the bars pack up with snow and ice pretty bad.
Some one could correct me, but I think the Mopar step bars are 4" and look similar to the Raptors, not nearly as nice though. The ram is a big truck so I wouldn't worry about getting too big of a bar.