Have you tried them both in person? Usually it becomes very clear what you're drawn to when they're on the wrist. I would choose in terms of what absolutely sings to you.
If you don't feel that way about either one, don't buy yet. But if you do have that feeling, you'll know which to get.
That said, there's no doubt the Datejust is the more versatile choice. I adore it, and the one you're thinking about is a knockout, but there are people who love the YM just as much. (If this had a poll I'd pick the DJ.)