@Martin Silverstrim
Hi Martin, unless you're buying in cash, I haven't found any conventional lenders who would finance a house to a LLC. I know some folks say you can deed it to a LLC after closing and not tell the lender, but that would give the lender the right to declare default, if and when they choose, and that's too big of a risk for me personally. Commercial lenders will finance to a LLC but the rates are horrible.
I don't know where you live but the transactional cost of maintaining a LLC is also a factor you need to consider. If you live in a state that's cheap to form a LLC and cheap to maintain it each year (annual registration fees etc), then it may be worth it assuming you can get around the lender issue.
If you can, diversify like what the others said. In my experience, the stock market has not really given great returns in the past 10 years even though we've had a bull market. The compounding thing hasn't really played out for my portfolio and I invested in index funds. So I wouldn't bank on that S&P 500 for 15 years. You'll be lucky if you get an average 4% return over 15 years. I think the real value of 529 is being able to save tax free and grow tax free, and if your State allows a deduction, you get an extra tax benefit.
In contrast, my real estate portfolio has done far better in cash flow and equity the past 10 years. I plan to use the equity in real estate and 529 to fund my kid's college fund.