Product? Marketing? or Sales? What would you choose?

Well except if you want to run a consulting business, you will need a product department.

How do you Differentiate Sales and Marketing roles at an early stage?

Definitely the product team. If you have a good enough idea, there are plenty of communities and marketing channels that you can use to get your product out there. The marketing and sales teams are multipliers on top of this. I suppose in this hypothetical, you could skip the product team by validating an idea using some sort of sign up page or no-code solution, but then you gotta play catch up. And I wouldn't sleep on design too long either — this will inevitably come back to haunt you if ignored too long.

A strong sales team is the only one of the options that can literally have a business built on zero dollars and with zero investment in the other two areas. You can find someone else's product and pitch it yourself and get commission-only sales people. Marketing requires money and product requires investment as well. And, all of you people who think you can create a product with zero marketing dollars or with zero sales skills have certainly not done it. There's too much noise out there and this would only work if you had a crap-ton of investment money to make it work, in which case it would be silly to go all-in on any one department. And, even then, I'd be really surprised if it took off by itself without some pre-established community or someone to go out there and hunt for you.