I would like to discuss, which are the superior instruments to trade M3 or BB taken into consideration buying power effect and total cost efficiency. For major equity indexes like S&P500, RUT and ESTX50 I have analysed the products which can be traded via IB. To analyse the total cost perspective (total costs => spread + commissions) of iron butterflies in the different instruments I have compared the following: 1) bid/ask spreads and the associated costs 2) spreads if mid prices can be achieved and the cost effect 3) cost of commissions in order to trade the identical notional value for each 4) buying power effect in a RegT account IMHO for S&P 500 the order seems to be: 1) ES (spreads No. 2, commissions 2, buying power 1) 2) SPY (best spreads => best pricing, worst commissions due to lowest contract size, buying power 2) 3) SPX (worst spreads, lowest commissions due to highest contract size, buying power 2 => the same as SPY) For a portfolio margin account it seems to be that even for large notional value trades SPY is the superior instrument to trade the S&P500 with an M3 or BB. The highly competitive spreads of the SPY outweigh the higher commissions by far. The effect is even more important when there is trade mangement necessary For RUT the conclusion is the same: IWM is way more cost efficent than RUT. ESTX50 options are almost as cost efficient as SPY (better than ES and SPX) but have SPAN margin which is the same buying power as the ES FOP. Which of the instruments you guys trade and why? Do you consider the cost efficiency to select the the instrument for your trades?