I prefer the Safariland Comp I. Simple operation...insert rounds, push on the loader's body. I have (unintentionally) drop-tested mine, including on concrete floors, and the rounds never spilled out, but the loaders still worked fine. I've read of people taking a loaded Comp I and throwing it against the wall without spilling rounds and still working.
However, if you carry in a split-six-type pouch, the release button rides on top of the belt. With enough downward pressure, the rounds could get released. While it's never happened to me, it's one of the reasons why I don't use split-six pouches anymore.
The biggest problem with Comp Is is that they don't work particularly well with the factory rubber boot grip that comes on most current J-frames. You can either switch to a different grip with a better relief cut or modify the grip with a Dremel tool, or file and sandpaper (the method I used to modify a set of PGS Hideout grips). Personally, I think it's worth the effort/cost to be able to use the Comp Is.