If I wanted to day trade the /ES using stop losses, how likely is it that it gets blown through on say bad news and I get filled at a drastically lower price? In this case is it better to trade SPX credit spreads?
If the market moves fast enough, very likely. Liquidity gaps can occur at any time and have become more frequent over the past 10 years. The alternative is to buy a put which you mentioned in reference to the credit spread, but this will weigh down your returns considerably.