Not just the multiplier, but exchange fees/broker commissions, as well. On IB, SPX is roughly 1.31¢, while the ES runs ~1.62¢. [so it's actually another 24% more. If you're piling $1k in commissions a week, it matters. If your commissions are $100-$250, it's a pizza.]
Ohhhhh, you'd THINK it'd...