To answer your question: "What's more important...a highly technical partner or a highly compatible and motivated one?" Hands down motivation over skill in my opinion and experience. I've seen one go the other way, or finding a way to get help, but I've barely ever seen the reverse. Also, one quote of Richard Bronson’s I find myself repeating often "Hire for attitude, train for skill."

I think your team might benefit from a 3rd, back-end/server side cofounder. Then between the three of you should be unstoppable with all bases covered. If you cannot find that then you're doomed with or without your non technical cofounder, unless you learn enough to be dangerous, build enough to be able to afford an employee who is a back-end person, who most likely will end up trashing all your code and building it from scratch. I'd sure hope you'd welcome that.

You never said what your dilemma was.