looking at the big electronic contract (ticker ZSPZ8) - liquidity pretty much matches up the liquidity in the ES in dollar notional.
It trades on top of ES contract when bid/offer at round or 1/2 increments, but slippage of 0.05 pt vs. the ES when the bid/offer is at the 0.25 or 0.75 ( which amounts to $12.5/ big contract or $2.5/small contract).
So it looks like commission + slippage in big S&P contract will be about the same cost as the ES contract (though there will be times when you manage to buy and sell at even/0.5 prices and it will be cheaper vs. the ES).
Haven't looked at the dow mini vs. big contract yet - but suspect that slippage will be negligible as the bid/offer in the mini is smaller.