A 31/2 definitely has more knock down power than a 3". Whether you'll need that extra knockdownvery oftenwill remain to be seen but it is nice to have it there waiting for you when and if you ever need it. I shoot the 3 1/2's and wouldn't trade it for anything. A 3 1/2" will give you more shot in your pattern too if you get it hooked up with a choke and shells it likes. My Rem. 870 Super Mag will consistently put over 100 pellets in a head and neck turkey silhouette target (equivalent tothe out line of your clenched fist and forearm)at 30 yds and 85+ at 40 yds and 45+ at 50 yds. That is a lot of shot in a small area at some pretty decent distances. I don't think you will find a 3" that can do that. Most times you won't need that kind of pattern but when you do it is nice that it is there. If you blow a yardage guess it is nice to have that kind of knock down power. Also if you buy a super mag you can always shoot 3" out of it anytime you want to.

The Simmons Pro Diamond Recticle is a decent shotgun scope for slugs.