When people say "I have no control over the stock market", they're typically referring to individual stocks. And they're right. I've seen individual stocks get manipulated time and time again. But the entire market at once? That's nuts.
Which is why it's best to have monies going into both, real estate and the company 401(k). Take the match (assuming you get one), invest the 401k monies into broadly diversified low cost index funds. And just leave it alone. There's 500+ years of historical data available proving that this is the single best method. Read John Bogle's book and implement that strategy in your 401k.
If you're invested in the S&P500 and it goes to zero, you me and everyone in between will be dealing with far more serious issues anyways. Do you think your real estate holdings will be performing any better than the market at that point?
Your duplex will likely be a glowing hole in the ground at that point, or we're fighting off a zombie horde.
Which, truth be told, sounds like fun. Definitely a good stress reliever at the very least...
Me personally, I'm contributing to my 401k only up to the company match limit. 1/2 of the money is going into the S&P500 index, the rest is sitting in cash.
It's sitting in cash for two reasons; 1) I can (and do) pull the money out in the form of a loan (earning me 4.5%) to purchase real estate, and 2) I have a feeling we're due for a correction shortly. Not as big as 2008, but not a small one either. When that happens I want to be in a position to load up on the S&P when it's at fire sale prices.
