As others have said, now is the time to be risky. If you think that startup will hang around for a bit (1 - 2 years) that will be more than enough experience for you to land the next job. I'm not telling you to job hop, but it is completely normally to only stay at a position for 2 years (or less).

I personally would choose the one that would help me grow more as an engineer and IMHO, you can get pigeonholed much more easily at a big corp than at a startup.