I Have a Grip and had a Revo Dualpack. So far I really like the Grip bag a lot. My main reason for getting it was to help with back problems and it has done that job very well. Sits up high and tight to my back and it feels lite as a feather. I can fit 18 discs in the main compartment and 3 in the putter pocket. Bummer is, I use to carry 22 (not including my putters) in my Voodoo bag. So I had to make some cuts in the number of discs I carry, but even at 18. It's a tight fit and I have to pull a few discs out to get to the ones on the side. The compartment opening could have been a bit bigger, but it's no were near as bad as the dualpack was. Quality seems top notch, but time will tell. The other odd thing I noticed, but it's a very minor issue. I think the bag is not wide enough by just a bit, because when loaded I can feel the discs agaist my back. Although padded I can still feel them, but not enough to be a problem. It just cuts down on the comfort and makes the padding a mute point.
This bag it soooo much of a better design than the Dualpack that it's not even close. I got rid of my Dualpack within a month. It's just a VERY poor design! The two biggest issues I had with it was that the top pocket would "fall" down onto my discs and make it a pain to get them out. Even after puting a plastic piece on the top of the internal frame, it still was a problem. Couple that with the small opening to get to your discs, it drove me NUTS!
Never tried the Mahal, but the owners sure seem to love it. So as far as backpack style bags go. It's the Grip for me, but Shane at Voodoo said there is a packpack style bag in the works and Revo is suppose to be doing the same. Also, I've owned several bags over the years (Innova, Phenix, Voodoo, Revo, Lat 64, and GB). Voodoos "bag" style bag is the best I've ever owned, but the Grip is as close to perfect for me (backpack style wise).