I looked-up the interest rate for an I-Bond bought in May of 2003 and calculated it's internal rate of return if cashed out November of 2016. It came to 6.78% annualized. My numbers were $10,000 to start in May 2003 and it ended November 2016 with $24,268.
Then I went to an S&P 500 CAGR calculator
and it said $10,000 (in January 2003) turned into $33,800 by the end of December of 2016. That's 9.09% Annualized. Not exactly the same months because the calculator only allowed selection by year.
So the S&P 500 got about 40% more money than the IBonds.
BTW, if you're looking at the rate like I am, in nominal dollars, then you'd not mess with inflation (they both have inflation in them). So I "pretended" to take one batch of $10,000 of 2003 dollars and put it in the bonds and I took an identical batch and put it in the S&P 500 with dividends reinvested. Then I compared what would happen if you cashed each out in 2016.