If I am out hiking through the bush, the tube fed is clearly more streamlined, and you are going to load it prior to use, so it doesn't need to be fast to load.
The mag feds stand out when you need a gun to go from unloaded to loaded in a hurry. For instance, I keep a mag fed Rem 870 handy at home, with magazines loaded with various loads. Slugs in one, buckshot in another and one with some smaller shot. This way you are ready to deal with bears, coyotes, cougars or small pests, all with one easy to load firearm.
I could see magazine fed shotguns being fast to reload in 3 gun competitions, although I don't participate in them. The REM DM is very reliable, always works but it does have 2 drawbacks- the magazines don't hold ammunition well if they dropped, and the magazines need to be pulled out and firmly seated in. Don't expect them to "drop free." Although with a bit of practice it's easy to deal with quickly.
Can't comment on the Grizzly Mag feds, I've never tried one, although I had a tube fed 14" one that worked as well as any other pump gun.
To date I've got a little over 1000 slug rounds through one of my 870 DM's without a problem. The other one has seen only a few rounds, but I did kill an aggressive bear with it this year. Point blank range, too.