For example a 5-Lot-RTT 2390/2340/2260 with Expiration at 28/29.Sept 17 for 0,70 each Lot just in the moment cost for ES 175 $ plus commision 28.20 $ Margin shown in the tradepanel of IB is 1416 $. cost for SPX 350 $ plus commision 26.66 $ Margin shown in the tradepanel of IB is 13008 $. You can try it in your account at TS without sending the order. I think at TS in the tradepanel there will the commisions and margin be shown. Then you can compare them yourself.