I think preparing for the worst is a good way to avoid it. Worst case is going broke in a business that doesn't pan out. And actually, in bankruptcy you can keep your Roth IRAs (up to a certain amount). So I'd contribute to the Roth IRA: tax advantaged savings, and money you keep even if your first business doesn't work out.
Have you considered working in a small business for awhile before starting your own? You could learn how to handle things that might otherwise sink a new business. That's one way to reduce risk.
If you go broke, you lose the house. I think it's better to have money in the bank ready to pay the mortgage, rather than spend that money chasing a small return. Also keep in mind the mortgage payments include interest, which is tax deductible. So the actual return on pre-paying is less than you expect, since you lose some tax benefit.
Pay the Roth, keep the cash, wait for your business to succeed before spending cash. A business without cash is no longer a business.