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.