I bought the Garvee 4 prong 36" jobs because I have several dozens of 5 - 12" hickory logs to move. I've managed to move much, much bigger stuff with them, but, they are not as easy as the pictures make them look. I cannot, for instance just drop them on a log and have them open, slip around the log and constrict to pick it up. I find that you really need 2 people to work them properly: one to operate the tractor and the other to open, set and close the grapples, then open and remove them.
I dealt with part of this by rigging a 'trip line' on the bottom brace of the grapple which I operate from the seat of the tractor. I use it to force the grapple open then drop it on the log and release the trip line which allows them to constrict on the log and let's me lift it. It's not easy, but works OK. Once I move the log, I need to physically get off the tractor and open the grapple up to take it off the log and go back to work. Having 2 people would make this much easier and faster.
Having said that, once you get the log into the grapple, the only way to get it out is to drop the load, get off the tractor, open the grapple and physically remove it. I've never yet had a log twist, turn or spill its way out. Once the grapple pinches, it's there until you take it off.
Here's the unit I bought: