With Shotguns I think ability to fully reload the gun is as much as practical due to the size/weight/bulk of the ammo. 4 to 6 rounds.
While I generally agree with Tom Gibbons theory shotgun rounds as being a “serving “ hitting with that serving, multiple opponents, or the potential need to use fire for suppression to cover movements etc means I want to be able to feed a good catholic family.
I’ve had the best experience with side saddle types. The Mesa and Aridus versions bring better than the original side saddle type. I like the idea of Velcro cards but have had rounds drop out of them while working. The Aridus version seems to be best.,
Butt cuff is better than nothing. Not a fan of ammo on the sling - it becomes a pendulum if the sling is loose and is not consistently placed making for even slower reloads.