It comes down to two questions in my mind.

1. Is there a time to market issue with either project? In other words can your project be put on hold and still be viable if you come back to it in 6 months or a year? What about their project?

2. There is the question of control? Do you want to control your own destiny or not? If you do you stick to your own project if you are okay with seeding control of some of your future directly to others you have to consider what they are selling

Once you answer those two questions the choice should be self evident.