I have had and used both. If you are using TSS just pick which one you like the feel of best. I think the backbored SX3 will pattern better overall. The SX3 I feel like is a better all around gun, I like the speed load, recoil reduction, pattern, no Benelli click, and all that good stuff. That being said for turkey only I pick the M2, mainly because of its weight. I put in a Wolff recoil spring to help eliminate Benelli click.
Can't go wrong either way with those guns!