you need to understand, despite what it looks like, the large institutions arb these things heavily. If the ES emini moves, almost always and quickly, so does the SPY. Now, Nasdaq (QQQQ) and Dow (DSU9) or other dissimilar moves somewhat differently, but the same index predominantly moves like itself because it sets the same companies
This is why economists never pick up money from the ground. "If it was actually there," they claim, "someone else would have already picked it up."
in response to your original post... yes. execution speed and costs are everything, especially in the index ETFs, as is margin cost when holding a position overnight. SPY has been hard to borrow recently and can be very costly with just a medium sized position. i recently posted a question about synthetic futures in the options forum...doing a combo trade vs SPY may allow you to lighten up overnight. got a helpful response. if you're doing any of the index ETFs vs. futures now, how are you valuing the spread? moving average, just looking at mid-market of what you can put on, etc? FYI, LIFFE US recently announced an incentive program for traders willing to make markets in their gold and silver contracts vs GLD and SLV. their contracts are very thinly traded though.
trading a tight relationship like that is very difficult, similar to trading a/b stock pairs or mergers. if the markets are moving fast, execution becomes an issue with slippage on either side of your arb. You will have a high cost of carry and with volatility dropping steadily arbs are becoming harder to find that are large enough to capitalize on. My personal preference is to take more risk with a speculative arbitrage position with a strong fundamental bias and a technical analysis of the current range and historical ranges of correlated stocks or indexes.