That sounds interesting, so are you saying ES has less slippage? What does slippage mean here and how do you measure it?
IIRC and correct me if I'm wrong, SPY typically has a 1c spread during market hours which as a percentage is smaller than 1 tick on ES. If you get price improvement on SPY then the real spread will be even smaller.
SPY: 0.01/270 = 0.37bp
ES: 12.5/(50*2700) = 0.93bp
Buy 1 ES @ $2700.125... $135,006.25
Buy 500 SPY@ $270.01... $135,005.00
But there is more slippage in the SPY...
1. The time it takes to execute... that's not just "exchange" time.. it's also the time it takes you to decide to make the play, enter the order on your platform, and get it off to the broker for execution. Of course the market may move in your favor during that time.
In the ES, you can "instantly reverse"... long to short, and vice-versa.. with one click of the mouse.
2. Settlement. Once you sell, you can't do anything with that money until settlement. Of course you can/may use margin, but pay interest to the broker for doing so. Otherwise, you're locked out of the marketplace for 2 days. Of course you can play only a portion of your capital to have cleared funds for the next play while waiting for settlement. For comparison purposes I'm assuming you're committing ALL of your capital and don't have that option.
Besides, futures are 1256 contracts and get a bit of a favorable tax treatment... though that's not a slippage issue.
The daily $$ handle on the ES is about 10x that of the SPY. I'm sure that if there were an overall advantage to the SPY, it would be the one with the bigger handle.