SPY is extremely liquid, penny wide, and low commission (actual rebates for add liquidity?). So why not SPY instead of the mini?
1) Futures and equities have differing tax treatment for US-based traders, which can make futures trading more attractive 2) Futures can offer higher leverage, if that's what you're after...
Commission: At same values, good broker, the ES is around 5 times cheaper. Trading hours: 23.5h for ES, 8+ h for SPY Margin req (approx): 25k cash buys you 100k value of SPY 4k cash buys you 100+k value of ES The only downside to ES is that the ticks are slightly larger (0.25) but you get over it with a little experience.