Another possibility that has not been mentioned is the S&P500 "big" traded at the CME. It's 5 times the size of the ES and 25 times the size of the SPY, so that would keep your transaction costs down.
It's also pit traded, in a futures options pit (which is what I'm used to), so the comments and suggestions I've made in this thread are probably more accurate for it than for the SPX.
If the firm where you have your account agrees, you can definitely call into the pit and talk directly to a broker. IB might not let you do it, but the smaller futures firms will if your account is big enough.
It's also pit traded, in a futures options pit (which is what I'm used to), so the comments and suggestions I've made in this thread are probably more accurate for it than for the SPX.
If the firm where you have your account agrees, you can definitely call into the pit and talk directly to a broker. IB might not let you do it, but the smaller futures firms will if your account is big enough.