I still haven't found the perfect solution.....
The general rule in rigging is to use the least amount of metal possible. There is no reason to have more connections than you need. That leads to more failure points, and more flying metal if something does fail.
I generally use just a single shackle that is directly in a thimble on the winch line and the two ends of the tree strap.
I have a few friends that run the self-closing winch hooks. I like using them. They are really simple as long as you hands aren't too cold or your gloves aren't too loose. The same thing goes for shackles. They are pretty hard to use with cold hands too. I think the biggest advantage to the safety hook is that you can't drop the pin in the snow and have to dig for it.....
I also think the safety hooks ( self closing kind ) are a little faster than a shackle.
Also, always remember if you have a hook with a non load bearing gate to always try and attach the hook with the unsupported side pointing up. The theory is, if the hook was to fail it would go towards the ground, not up into the air. This doesn't always work out with the rigging, but its good to remember.