E-mini's are traded nearly 24 hours during the weekdays on the CME Globex system, whereas the SPY is only traded during normal and extended US exchange trading hours. Naturally, after hours on ES do not have the same liquidity as during the day, nevertheless, from a risk management perspective, this adds value.
My recommendation to those that have a portfolio of short options is to run the "Global risk analyzer risk report" which gives you an estimation of the impact on your portfolio with a number of Standard Deviations applied. This could potentially give you an idea of the level of risk you are running. Options sellers need to think in terms of the one off event.
Those of you who traded Option on ES in August of 2015 know what "out of the money" does to your portfolio.