Probably the biggest reason I would join Google is for the brand name (and, I think I will enjoy the work and the people too). Having Google next to your bio gives you instant credibility (like the quick summary of the co-founders above). The cofounders already have that, but I don't. The question is is it important enough to hinge a big decision on?
And as for why I want to live in the Bay area, I spent the last 4 months in SF and loved it. The weather, the people, the whole vibe are awesome. Plus my brother is now working in LA so we'd be close, although my parents are in Richmond, VA so I'd be closer to them in Pittsburgh.
Looming question at large: Is a brand name like Google necessarily better than being the first biz-dev hire at a promising startup? The startup isn't "sexy" like an airbnb, but they're already profitable and are getting more incoming leads each week which is great to see.
I spent 10 years in the Bay Area and now live outside of Philly.... I prefer it here. But, I believe that your 'roots' are where you are surrounded by people that love and support you. I would ask you why you've always wanted to live and work in the Bay Area? What is the attraction? Does it make sense to your other life goals?
Ultimately, I would encourage you to chase your deepest dream while you are 25 - it becomes harder as you get older and create more responsibility.