I actually hate them both, but would opt for the cantilever because it is part of the barrel, and on most bird/slug guns there is too much play in the barrel to do otherwise.....I mush prefer a fixed barrel gun like an Ithaca Deerslayer II with the tapped receiver.
If you want a barrel that does not have a lot of movement drill and tap(fine thread)the ring that circles Magazine tube 2holes 60 degrees apart use allen hd set screws with no mar/plastic end feature and adjust so it is snug.
If you are not handy get Gunsmith to do it really improves accuracy on 870's.
Cantilever, no argument. the cantilever setup allows the scope and barrel to maintain alignement after removing the barrel from the shotgun. I've used this setup for quite some time and it's never needed re-sighting.
Cantilever is OK for a scope but it was to high for a Bushnell Holosight Red Dot...Best to use a Weaver Mount and hopefully your shotgun is already tapped.. if not...Your going to have to get it tapped.
Saw people using saddle mounts for Holosight type red dots.
Don't know if they worked for them..
However if the saddle mounts are high like Cantilever then probably not good for SOME red dot equal to Holosights..