The870 Express Super Magnum Turkey does have a 3 1/2" chamber just like the SPS-T.

It looks like there's nothing special on the SPS-T that's worth the additional money. I already have two 870 Expresses and have the Super Full Turkey choke already. That won't be an expense. I guess I'll just go with the Express Super Magnum and spend $20 on a sling.