Every move is a gamble, you have to guess the odds and go with your decision. I'd probably worry about working extra hard for a company that isn't yours though, especially a startup. Been there, done that, the very second they run out of money they'll treat you like a leper.
Yes, that's the worst part. If you push so hard and eventually it all ends up at a dead end.
What must be done in that case?
I don't think there is a simple answer to that.
What would be the worst case scenario ? Do you own your house ? Do you have enough savings to handle a few months without income ? Can you get unemployment benefits in your country ?
I know that in Europe even if you fuck up big time you usually won't en up in a bad situation (job security, unemployment benefits, social welfare, &c.)
If I take 2nd opportunity then I will lose my long established networks from current job. By losing I mean won't be able to as much connected and use the benefit of networking. Yes there's a chance that I get better circle in 2nd one but who knows ...