I agree, I love working on hard problems. Sort of. What I've found is that I tend to make decisions for a variety of reasons, and am most decisive when I have a either broad convergence of motivation, or a singularly powerful one.

If you're asking yourself wether to pursue a job at google or starting a startup, you'll want to try enumerating as many reasons as you can for each one. You won't likely get a definitive answer, but the excercise will get you thinking about what other things are important to you. If you're lucky, one will turn up to be the obvious choice.

In my case, I have slid technically to the point that preparing for one is the same as preparing for the other.