I have all three. I prefer the Contender, largely because I find it easier to get discs at the edge out, and because it has more storage at the top with the external putter pocket. You give up a little of the side storage (I like putting my approach putter in one of the side pockets - the second runs have a zipper there for storage which just about equals the Grip's) but I like the convenience. And the top storage area remains large.
The Golf Mahal I consider a pack for a day when I'm playing a BUNCH of new courses. I don't need to carry 32 discs or whatever, so if I'm going on a longer trip or playing a lot of new places where I'm not sure, I'll pack a few extras of my favorite molds. I've never been one to pack heavily (extra clothes), but if you are sensitive to temperature ranges and like to take a sweatshirt on or off, the side pouches (or one of them) on the GM can obviously take an extra pair of clothes.
The GM stores more, but it's not as much as you might think, because the "top and side storage" on the Grip and the Contender are just moved down and to the sides of the GM.
Lastly, I sense no real difference in comfort. Sometimes the GM feels a bit better, sometimes the width makes it a bit more of a burden to put on. I often walk with just one shoulder in, so the GM loses out here as it's really built for a bit of a longer walk. It's not as "easy on, easy off" as the more traditional backpack style bags.
I like the rain covers in the Grip and the GM. The Contender is waterproof but you've still got an external putter pocket. Still, if I cared that much, a plastic sheet would work in a pinch.
I know that probably doesn't help you, since I basically said I like all three. Between the Grip and the Contender I slightly prefer the Grip (the storage simply works better for me, and the ease of disc access at the edges), but between the Grip/Contender and the GM, it's really about what you want to do, how you play, the plastic and stuff you carry, etc.
I have the Catalyst on the way, too, because I'm looking for a slightly smaller bag than the Contender/Grip without the discs rattling around. It'll probably replace my Fade Crunch Box.