Depends on what you want to do. Other factors might rebut the presumptions below, but I would offer the general piece of advice:

Go to Stanford if you want to be an engineer or would like to do a startup someday - your peers and the culture at Stanford will help you learn more.

Go to Harvard if you want to be a banker or go into finance. Plenty of folks from Stanford go into the financial industry, but again, your peers and the culture at Harvard will probably help you more.

Obviously, you have two great choices, so no matter which school you choose, you'll have plenty of opportunity to learn.

There's also a great Quora post that analogizes Stanford, Harvard, MIT, and Caltech to the Hogwarts houses:

<a href="http://www.quora.com/Choosing-Colleges/How-does-a-star-engin...;

Disclaimer: I went to Stanford and majored in a non-engineering discipline.