I think both are reasonable paths to wealth. I see more risk in Real Estate, you end up liable for things you cannot know about or predict. Your investments are typically far more concentrated in a single city. Government shuts down the space shuttle program? you live in Cleveland and all the plants close? I'm sure there were quite a few landlords who had nice empires in Detroit, Cleveland, etc... who ended up wiped out.
For the additional risks in real estate, you should earn higher returns, and many people do.
So the question is: are you willing to take on some additional risk, and some additional work to earn those higher returns? Personally, I am not. If my situation were different - I would be. I take great comfort in my lack of leverage, and my lack of tenants or previous buyers, my risk of being sued is dramatically lower with my choices - and that's worth a lower return to me.
The survivors on here, who are really good at their craft, are blowing away my returns - and with their knowledge - they are mitigating their risks, and hopefully buying insurance where they can't.
@Matt R. while stocks may have historically outperformed real estate as an asset class, the stability of house prices allows for low borrowing costs, and a more typical higher level of leverage than stock investors. So given a slightly lower return with much higher leverage, I would think the average return for real estate investors is higher than stock investors. But its with the additional risk of that leverage.