I think a general concensus is its 100% valid to have both... but if only one, personally, PAM's first. You're use-case may be different though, working with semi-modular as well. It'll kind of depend on your setup. If you already have a solid clock and distribution plan, maybe I'd get the o+c? Again, personal opinion/grain of salt... FWIW, I don't own an o+c, but its a module I've researched/considered quite a bit.
Ultimately, I would caution aginst considering them 'similar modules'. While there is overlap, their strengths are very different.