There are a lot of differences between the S&P and the e-minis because there are many e-minis for different indexes like the DJ, Russell, SP etc. But if you asking if there is a difference between the big S&P 500 contract and the mini-SP 500 contract beside margin differential then the answer is that they reflect market price action the same. Many track the S&P500 (full contract versus mini) to better follow what the institutions move in volume during floor hours to determine support and resistance levels.