I'd save the money on the 3.5" shells and shoot regular 3" if it were me--especially if you're going to be shooting hevi.
I have said this here numerous times, so if you have read it, bear with me. When lead was legal, we would shoot ducks with 410's and kill them. Granted we're hunting out in flooded timber and shooting working ducks in your face, but none the less, it was with a 410.
3.5", in my own opinion, is absolute over kill for ducks.
I'd probably go with the 6's if it were me personally as I like to take my shots within 20 yards. Then again, I just shoot regular steel; 2's and 4's.
Ultimately, it's up to you and how you shoot that is going to decide what you need to buy.