You mean futures? Or the cash index SPX and XSP? For futures it is mainly the multiplier, big SP is $250 per point, e-mini is $50 per point. Although there are some other differences as well, check contract specs on CME's website for more details. For SPX vs. XSP, it's just that XSP is 1/10th the size of SPX.
Unless you trade huge size and e-mini liquidity is not enough for you, you should stick to the e-mini (ES).
Even though I responded to the question, I actually agree with you. Many times people ask questions like this despite the fact it would actually do them more good trying to find an answer themselves first and only if they cannot do so then ask for help. But a lot of people are just too lazy for this.
hahaha... sorry, next time i will find the answer myself. If i can't find then i will ask you people. anyway thanks for your help.