I'm starting this thread instead of posting in another that this topic came up, so I don't make it get off subject. The question wasn't asked of me, but I thought I might put some thoughts down for disucssion. Here's one reason for trading the SPY: (For argument's sake, let's say that we are talking about 500 shares, so 10 cents SPY = 1.0 points ES). The finer resolution makes for less slippage on entry. If you watch the ARCA or INET/ISLD books during the day, you'll agree that for much of the time there's a penny spread. Now this is more important for someone who trades frequently. Let say that trader takes liquidity. So, for each trade he or she pays the spread. On the ES, that's 1/2 point. On the SPY, it's only 2/10ths. This can add up over many trades. On the other hand, the SPY is much more expensive to trade than the ES. At 1 cent/share commission, a trader pays $10 RT for the SPY trade (assuming that the clearing arrangement doesn't also require SEC fees to be paid--another $4.90 in this case--or rebate/charge for liquidity) while most ES traders will pay around $5 RT. So, the SPY trade is 2 or 3 times more expensive. But, if you count the savings in slippage of 3/10ths, that's worth $15 right there. So, my point is that without the CME's changing the ES resolution to be more like the SP (1/10ths), the two are pretty much a wash.