The "pro" tube mag argument is already well spoken and all points are valid.
The mag fed argument has several twists.
The main one is good luck getting magazines, neither one has any and this is the biggest pain the @ss going!!
Plus if you can actually find one for the Valtro, it's a hundred bucks. Each.
Other than that IMO the Valtro mag system is nice because it holds 7 rounds but sucks because it has to roll into the reciever kinda like a AK. The Grizzly sucks because it only holds 5 rounds but rocks because the mag inserts straight up kinda like an AR. So in my opinion the Grizzly is quicker to reload but you have to do it more often, depending on what your doing with it it may or may not matter.
My experiences in IPSC have taught me that in the walk through you are planning your mag changes while moving, so in a timed event a properly set up mag fed will be quicker than a tube gun if the shooter is properly set up and familiar with his equipment.
If your not in competitions the ability to quickly and safely load and unload your firearm is better with a mag fed, no mag in the gun and the chamber open is an obvious and easy thing to see safe even at a distance. Tube fed gun, did you/he get them all out...
Personally I think the only reason the mag feds haven't been more visible in competitions is that both systems currently available have shortcomings and exactly how could one compete with one when the suppliers can't supply magazines.
go ahead, ask me if I'm bitter over the magazine issue...