Daniel answered most of your questions, but I would only add that one can probably get up to 60+% of their snatch max (two hands) with one. You are still using both legs and your whole torso, so the weights go up pretty easy. For me, the "catch" is the issue...especially with a barbell.
I have an Olympic Dumbell that rotates like an O bar. It didn't cost very much and I do all my one hand lifts with it...and Waiter Walks, too. I found it helps to keep either a 10 or, at most, a 25 pound plate on the inside of the weights and then add heavier plates outside of them. Big plates don't allow your wrist/forearm to "stay in."
The upside of the dumbbell is that you can go heavy and not break stuff. With a barbell, you crash the weights a lot. Fine if you are in a park, not fine if you car is parked next to the platform.
I find one arm snatches much easier than one arm cleans. I have gone fairly heavy with one arm cleans, but the bar really whacks you coming done. The snatches are harder to hold at the top, but easier on the bodywork.