If yo ucan handle a 3.5" shell...I would go with that because that shell has more pellets than a 3" shell does. As far as shot size goes with different chokes...you are going to have to expierement with what works best from your gun. Most people seem to think that a third party choke, such as the Primos Jelly head or the H.S. Strut or the the Mad Maxx, pattern better than the chokes that come with the gun. I would get one choke and try different brands of ammo with different shot sizes and see how your guns performs. If you are satisfied with your results, then great! If you aren't satisfied, then you can go and get a different choke and repeat the process. I use an 870 SPS-T and I use a Primos Jelly Head with Federal Flight control 3.5" #5 shot and I am getting good patterns out to 50 yards. But what works for me may not work for you. Best of luck this up-coming season!