FWIW,I traded with IB and decided to set up an account with TOS as I am tired of paying 6 bucks on butterflys over at IB..But here is what i found,and if I am wrong,happy to be corrected. If you are trading a spread,TOS routes the order to the exchange with the highest bid/lowest offer..Great if you are a market taker..However,if you split markets,i.e enter a .35 bid for a fly that is quoted .10 bid at .70,your order goes the exchange with the best offer,but stays there.The order is not rerouted to another exchange if a .40 seller comes in on another exchange.In fact,a seller came in and offered my spread on another exchange at .35 and I was not filled.I only knew as I was watching my spread market on IB...If I am not mistaken,IB reroutes spread orders to the "best" market....While I love the commision structure over at TOS,I do not like that method of execution..Any one have any thoughts on IB vs TOS and the execution of spreads?