As an adult you start making decisions that will affect the rest of your life. Children have their decisions taken for them.
If you take the decision of going with the startup it will be one of the worst experiences in your life. If will be daunting, it will be taking risks every single day. At the same time, you will be making a contribution. Even if you fail you will learn things you will never learn in Google. If you success you will be free with freedom few things will ever give you.
If you go with Google you will be safe, isolated of the mundane problems most people have and you will be able to focus immensely on very interesting problems sharing your time with some of the smartest people on earth. On the other hand your creative and independent muscle will almost disappear, atrophied by not using it, as the big organization takes the load from your shoulders. Over time, it will make your shoulders less strong.
The decision is yours to make. Write down on a paper what your values and priorities are in your life and how each decision will affect them.