I looked at both, and went with GC for the price difference. The main difference I could see was the mounting base. GC uses a piece of angle for the base and the thumb uses a heavy flat steel base. I added the channel to reinforce the top of the bucket, so the GC's angle base worked out well for me. I think if I was going to bolt it on, or weld it to a flat surface, I might prefer the base on the thumb. It did seem like the angle base let you play with the open/closed positions a little more than a flat one would have.
I did have to grind a notch in the top cylinder mount bracket to keep the base of the cylinder from hitting in when closed. No big deal, and plenty of strength left in the bracket. The folks at GC are very helpful when you give them a call, and I was told if you have a problem with a part that they will send you out a new part. Seems like it was a 10 year warranty on the grapple and 3 on the cylinder. I did manage to wad mine up like a pretzel after a few hours use. Keep in mind it is on a larger cab tractor and I was pushing up some big stuff. I somehow managed to get some sideways pressure on it I suppose and the grapple arms bent all to heck. With the long nose on the Belarus, you can't really see what is going on with it. I pushed up a big pile, rolled my bucket back, and saw the damage. I just am not sure what happened. It seems like it would be really strong with up and down pressure, as it was designed for, but it won't handle side pressure. I don't think any of them would though, and I believe if the grapple arms had not bent I may have damaged something harder to fix. I took it off and after some time in the press it is functional again, but not nearly as pretty as it was. I have not contacted them to see if they would warranty that or not. I'm still trying to make up my mind if it was operator error or a problem with the grapple. I wish I knew exactly how it happened.
Neither brand is gonna be as good as a dedicated root grapple, but they are also less than a quarter of the price. I spent more on hydro, and channel iron than I did on the grapple, so consider what it will cost you to get it working. I ran 3/8 line, but 1/4 would have worked fine I believe. Only other issue I can think of is GC told me it would all be pinned and it had bolts mounting the grapple arms to the brackets. I emailed them about it and they said they would get me some in the mail. I have not seen them yet, but it has not been that long so they may be on the way.