I am a lefty but use right handed pump shotguns, no problems. Re-sale is much better for right, if that is a concern.

3 1/2" 12 gauge shells didn't compare to my 3 1/2 10 gauge in terms of range. The main reason I bought a 3 1/2" pump was I thought more pellets in the sky would helpwith wary geese and reducing the cripples. However with Kent Faststeel, etc not sure it is really required, especially over coys. I haven't hunted birds as much as I once didbut when I do3" Kentfor waterfowl & 2 3/4 for uplandis all I require. You may want to try posing this question in the waterfowl forum though to see what the thoughts are for 3 vs 3 1/2".