Get a good lawyer. Really it boils down to this: can you build and market the product without their assistance? Will you find angel / seed / VC funding on you

Get a good lawyer. Really it boils down to this: can you build and market the product without their assistance? Will you find angel / seed / VC funding on your own? Can you bootstrap? If those answers are yes, I'd say do it on your own. If they are no, your "decision" is already made for you, really. Be careful that they know what your intentions are. That can help stay out of troublesome situations. If you intend to be a vendor, then make sure they know that you want to sell this elsewhere. If you're under contract, or a work-for-hire, they probably own the work. And make sure to consult a real lawyer on any contracts.