Since both frameworks are pretty new and not very popular (yet!), I don't think anybody has taken the time to do an extensive comparison between the two. I don't even think the final set of out of the box components is determined by the developers themselves yet, active development is still going on.
As for compatibility, hybrid apps run in the native browsers of the devices where they are installed on. Both frameworks need CSS3, so old phones will never be supported by either of the frameworks.
The OnsenUI-tag here on StackOverflow is the only support OnsenUI offers (currently), and at the moment of writing there are 0 questions/answers. Ionic has a very active forum on their website + some questions/answers here on SO.
I think having an active community backing up a framework will eventually lead to a better framework. Therefore I'd go for Ionic. Personally, I find Ionic's standard-design more appealing as well, but you should judge that for yourself.