"Working for a no name startup so that your future HR looks at your CV and doubt you do anything much" vs "working for Deepmind so that your future HR says wow the moment they look at your work experience section". Yup this is a no brainer. Unless you have the ability to carry the startup, there is no reason to take a dangerous risk like that. Moreover, startups won't let you work on what you want easily because most of the time they need more people to work on the main products.