I would recommend The Lean Startup by Eric Reis. I wouldn't want to try and generalize the book, but essentially you propose a hypothesis, and then test your hy

I would recommend The Lean Startup by Eric Reis. I wouldn't want to try and generalize the book, but essentially you propose a hypothesis, and then test your hypothesis using a Build/Measure/Learn feedback loop. This creates an environment that trains you to validate your ideas/approach, which in turn facilitates the most effective use of your (and your team’s) time and resources. Anyway, getting back to your question, I would do both concurrently and use the feedback from your customers to validate the features / functionality you build into your MVP.