SPY vs SPX$

[sawtooth500 33]

I know that SPY is an ETF and with it we get volume whereas SPX$ is the straight index. I also know that they do not have to be exactly the same, but for the most part they are similar. Today (June 10) it seemed like sometimes they were as much as .3%-.4% off, and SPY seemed to lead SPX$ since SPY is traded and SPX$ is not directly traded. 

Is that why we can use SPY as a leading indicator of what market sentiment is, and is it generally accurate that SPX$ will follow what SPY is doing?