answer would depend on your current background and what this startup does. From your wording it seems you are weighing it in terms of money. In that case it is hands down google as it is already paying more, is considerably less risky and will give you brand recognition for future employment. I think you should weigh your options in terms of you future goals rather than money. I would rephrase the question as Which step will help me realize my goals faster and go with it.