Do you have an idea worth submitting to YC? Do you have a good network of people to mine for cofounders?
I'd say that if you've got other people to work with, and an idea to pursue, then try your hand at it and go after YC and similar programs.
If you don't, I'd pick the opportunity that will add the most valuable people to your network (other people who want to do start ups, people who complement your skills, people who could become your best friends/people you'll be willing to go through hell with). That's not necessarily a startup, something like APM at Google would be killer for your network and experience, and bigger companies => more people to connect with.
Final pro-startup caveat: If you find the right people to work with, join a startup. You're only 20, and you're not short on cash. There might never be a better time.