In the decision making process, neither you or your co-founder have a clear vision of the future (unless you are fortune-teller, which I hope is not the case); so ultimately both of you have to resort to external validation of your hypotheses.

Similarly in my personal experience, my co-founder and I had long debates regarding the product road-map early on. One month later, when we had a number of early adopters, a short email survey to few of our early adopters or a 5 minute view of our analytics tool would resolve most of our debates.

You can hire a freelancer (e.g. from freelancer.com) to write a simple web program to do that. Shouldn't take more than a couple hours.