ES all the way. Slippage, capitalization, leverage, sheer liquidity, the fact that the market trades 23 hours per day - all are superior in the ES.
Biggest Reason?
Once you start making some money, there is a serious tax rate advantage to trading ES compared to SPY. SPY is treated as ordinary income, and is capped at a rate of 35% ES is treated as blended capital gains, which is capped at a rate of 23%. Furthermore, if you trade futures, you are exempt from Self-Employment tax. No kidding - the Chicago cartel. If you trade SPY as sole income - guess what, you have to pay SE tax.
I have personal friends trading only futures who were once Don Bright's bigger (biggest) traders in Vegas. Why did they leave ? Taxes.