I just answered this question in a private email, but just in case anyone else will benefit, I might as well put my reply here too.
I don't blame you for being confused about the difference between the G.R. guy and the Magnum. They both have the enclosed track on the butt, cut for a 5/16" shaft, but the difference is in the ballast. The Magnums have lead added to the butt, and then an extra strip of wood added to the lower surface of the butt in order to balance the lead and keep the buoyancy of the gun proper.
I had never seen the G.R. gun until a month ago when a friend got one, and frankly I had always had doubts that it would be heavy enough to handle that 5/16" shaft and 5/8" bands without having problems with recoil and accuracy. I got into a three-way email conversation with my friend and Daryl on this subject, and my impression is that Daryl agrees with me. He advised my friend to order the G.R. with a 9/32" shaft.
You may be concerned that the track is too big for 9/32", but it really is OK. It still captures the shaft, and in fact I have a 9/32" shaft in my magnum right now and it works fine. And the enclosed track will let you power it with three 5/8" bands and still prevent shaft whip. My oldest Wong is a 55" regular hybrid with 9/32" shaft and an open track, but I use three 9/16" bands on it because I'm afraid the shaft would noodle if I used more power.
Some people seem to be concerned that thin shafts won't penetrate big fish, but I've had good luck with them.
Here is a fish that I took last Tuesday with that regular hybrid, and it toggled the slip tip on the far side of the gill cover. Of course the Magnum would have done it at a longer range.
So I guess the bottom line is that if you want to use a 5/16" shaft and have the most stable gun, get the magnum. But if the extra cost is a factor, get the G. R. and use a 9/32" shaft.
I hope Daryl will correct me if I've given bum scoop.