If we're talking about 37" tires and smaller, hands down Mud Grappler. I've never been fond of the way Trail Grapplers have that annoying pull to the left and I have found their side walls to be weaker than I prefer. The Mud Grapplers in that size are much tougher and hook up better on the rocks. In a 40" tire, that would be a lot harder for me decide being that the Trail Grappler is a totally different tire at that size. Still has the annoying pull to the left but it feels and behaves more like a 40x13.50 Toyo MT in that size only with better hook up on the rocks and much more aggressive shoulder lugs. Of course, if we're talking about 40x15.50's, Toyo MT and Mud Grapplers are the only ones in the family that come in that size and I would have to go with the Mud Grapplers.
BTW, here's a shot of Rubicat wearing Moby's old shoes and out in the snow.