Wow from the sound of it all, sounds like staying at a wall st job with a decent pay/bonus is better than joining a startup, except for the possibility of working on something kick-ass.
i left my electronic trading job, which had great compensation, to move to san francisco and work at a startup. i knew i'd be taking a pay cut, but i did it for the experience, so i could be better prepared to do my own startup.
i'm glad i did.
Are you involved in customer development, talking to investors, making connections as part of the startup? If not, there's not much upside.
I'm really sick of companies that think I will take a 30-50% pay cut for a foosball table and Nerf-gun fights on Fridays.
2) You can gain valuable experience by starting your own company. There are plenty of blogs, articles, and sites like HN that can help you along the way.
In my experience the best reason to work for a startup is the freedom it gives you. No "architecture board" telling you what technology you can and can't use. No reprimands when you stray from your assigned niche.
I'd definitely advise against taking an insufficient salary and thinking the equity you may receive in exchange will cover the loss. The odds of that aren't really any better than gambling. Fortunately there are plenty of early-stage startups willing to pay fair salaries and decent benefits!