Wow MBT is that cheap? too bad they dont take Canadians or I'd open an account. Anyways, I use IB and love them. ES is not nearly as fluid and tight as the SPY. I trade both but prefer SPY generally.
i trade ES, SPY, and SPX options.. Its more about position size to me.. thats more important to me.. I trade with IB and commissions are cheap as has been stated... if i'm going to calender spread i use SPX.. if i'm going for a 30 wide fly.. maybe ES .. a wider fly SPY... everything is measurable.. unless you quantify exactly how these SPY fills at such a lower notional value relate to SPX options considering the commiissions... for me at IB 10 spy flys are 28 bucks! so putting on a fly with SPX for 2.80 makes sense for me even considering the quote on quote bad fills.. Plus i let the market come to me and i position size small..