OK, that doesn't answer my question. You're taking different executions of the same idea. Obviously anybody that's been paying attention during the last twenty years or so knows that execution is more important.

But how do you vet the idea itself against other ideas? How do you know whether spending your time reinventing search is going to be better than building a new productivity app?

You're going to execute the best you can, assuming, again, that you're aware that execution is the biggest factor. How much, if any, effort and validation should go into choosing your idea?