Well to start with, all 3 are "open" blocks - in that you can just place the cable on without having to open the side of the sheave. I suppose they would be just fine as long as you have some tension on the cable. I kinda like the second block which allows you to use a choker chain as you attachment sling. The third block (6 ton self release) could be really usefull but could also be really dangerous if you are not paying 110% attention. When that slider pops the release you might get some significant changes in tension that could be "exciting".
They are a bit expensive for casual use but I'd probably get some if I was in the business. I have found snatch blocks at flea markets and such for from $10 to $40, depending on condition and size. Make sure the sheave (rotating center section)is firm and rotates freely - a wobbly sheave indicates a worn bearing (could be fixed - maybe). Also, and this is pretty important, be carefull of the size of the sheave and the clearances on each side of it (between the sheave and the body). You can ruin a cable, a rope and or your load if the cable pops out of the sheave and down the side of it. (I look at things more from a riggers view then a log puller - though I like to do both). Sake Mense?
