I prefer the Nordic mag extension mainly due to being able to use the same tube on different guns with the appropriate nut. I now have two Nordic extensions on shotguns, one I use for informal 3-gun matches and snow goose hunting and another gun I use for snows. I know a few that are satisfied with Sure/Cycle extensions.
I know several that use Sure/Cycle systems on their guns, I haven't seen a difference over factory but my experience has been mostly during hunting.