First thing first, for all practical purposes, startup stock is useless -- and especially if you're already thinking of quitting.
In my situation, I joined the startup right after college, and it has been almost 2 years for me as well. I was the first technical employee, and except one other employee (the second hire), I'm not seeing other people better than me joining either. That said, there are a few things to keep in mind:
- Startup is NOT the place to be learning technical skill, especially early stage startup. The big 4 will serve you much better than that (and I know this going in). However, one of the things I've learned over the last year is that there are more to it than just technical skill. I have no doubt that I haven't been learning anywhere as fast as I could have in term of tech skill, but I've learned other stuffs that might just be as valuable: from dealing with (less than ideal) team member, dealing with expectation (mine and others), being resilient, and a gazillion things that probably make me a better human. Startup is emotional, for better or worse.
- I remember reading a quote that if you can only learn from people better than you, it's gonna be awfully hard to learn anything. As you eventually get better, you will have to learn from everyone around you.
- Since both of the previous points are extremely personal and subjective, I think that the one most important point to consider would be this: do you think the startup have any fighting chance? If the startup grows, both of those previous problems will fix itself. If the startup have no chance, well why would you stay there anyway? In my case, I can't answer that question with either a straight yes or no, so I can't decide.