spy and /es are interchangeable and trade at a correlation of 0.99+/- , when they diverge arb traders come in and 'fix' it as both reflect a basket of securities.
if you're going to trade anything versus spy, my suggestion is to consider watching their correlation as some stocks and spy can trade at very wide divergences and never come close to intersecting.
ok, I'll ad that if you're a Tier 1 firm, there are real trading opportunities in these minor divergences, but, imo, the differences for most retail traders is not significant. please don't @ me because someone read an academic paper saying how much money they 'think' retail can make trading the arbs.