It looks like they do many of the same things, more of an issue if one does em better than the other. Onyx has worked for me, so maybe I shouldn't try to fix it if ain't broken
I've got a certain amount of trust in OnyX. I've found that the developer is very knowledgeable. And, he has always responded with an update within a day or two any time I've reported an issue in the past.
It's OnyX for me. I don't even have to try the other one to make up my mind.