1. The equity sounds all good and well, but you need to know the intentions of the founder — would they every consider selling or doing an IPO. If not, your shares will have zero value.
2. What space is the startup in? Do they have a technology where they are both building a moat and growing at a rate where they will be around in 5-10 years?
3. Can you live on 1/3 of what Google is offering you?
If you believe your startup is the next Dropbox, Airbnb, Uber, SpaceX or Pinterest I would say join that. The thing about these mega-companies (Google, Apple, Facebook, Microsoft, Amazon) is that they aren't going anywhere and if you made it through the interview already, I'm sure you can do it again. These mega-companies are filled with brilliant people so you will definitely learn a lot about running projects at scale. They are also filled with bureaucracy (unless you're on a skunk works team with no budget) which sucks if you're the type of person that likes to move quickly.
In my opinion, a great startup beats any mega-company any day of the week.