Congrats on the new gun purchase!!!

My first gun was a 870 3" mag express with walnut stocks. Never have shot slugs with it, but i have several friends with 870's 3 and 3 1/2" guns, some shoot smooth bore (pumpkin ) and one shoots full rifled cantilever barrel. It all depends on your preference. Full rifled should have better accuracy (and cost more), but remember your not going to get bolt action rifle like accuracy with either. I don't think that remington is offering the rifled barrel in thier combo either.

I agree with night hawk 223, get as many different types of slugs as you can get and try them all. Let the gun decide what it likes.

Also have some friends who have started going to 20 gauge for the hope of a flatter shot and less recoil (fast followup shots are the name of the game when driving deer)

Don't know your experience level, but be safe. It is the only way to be.