depends on what you want to do. catalyst has a proven history of big teams working on big projects and not getting in each others way. as well as one man teams working on small projects.
it's mature, well supported and powerful. there's a bit of a learning curve when you first start, but that's the way with everything. although it might be made worse by it's opinionated approach to the way you have to lay your project out, but not long into it you'll be thankful for that, as it helps keep everything clean and clear
mojo is more flexible about how the project is layed out, has async and websockets and stuff, but i've not used it, so i don't know the ins and outs so i can't comment
that said, you should try them both, see which one suites your needs best.