Internship: Unicorn Start-up vs Boutique PE

Depends what the exit plan is. IB/PE? Take the boutique. Unless you want to stay for the next couple years in the start-up world I would stay clear. Also, at junior level you almost will never have an impact or do meaningful work at a start-up. If you really like that world, join one after 3-4 years of experience elsewhere and you'll be able to have a more senior role.

I wouldn't do the startup either and I'm an entrepreneur. The skills you learn in PE are very, very transferable if it's a smaller operation BTW. I've worked with a few PE guys now from smaller ops and they have a ton of useful ops experience that they bring with them to any co. they get involved with. Exit opps do not really exist unless the startup gets absolutely massive.