Re: Invest in 401K or pass it up and keep entire pay?
A 401k and Social Security are not mutually exclusive. And you should, if at all possible have both. You have zero choice in whether to contribute to Social Security and just about as much confidence that the program as we currently know it will even exist when you and I become old enough to begin getting back what we have put into it.
A 401k is your own money and as such is always going to be better than trusting a government program to provide for you in the future. If you have an employer who offers any kind of match, you'd be an idiot to pass up whatever the matching percentage is. That's like getting a 100% return on investment the second you make the investment. You just need to make sure you know your risk tolerance and invest accordingly. If you can't stomach seeing the short term swings where your balance jumps by large amounts day to day, I wouldn't invest too aggressively. Typical advice is to invest more aggressively when you are younger and have time to build your wealth and move gradually to less risky investments as you age so your nest egg doesn't take a substantial hit right before you need it.
But as joe said, consult a financial adviser who can explain things to you in more detail so you can make the most informed decision for your circumstances.