ok FCM are like GNI RCGand GH ib are like advanced futures ascendtrading and linn. the FCm service the IB they pay for a leve of service. they IB set what price they chage after the FCm tell them the cost each one is differnt some good some bad some ugly some have great service some suck good luck and do you home work then you also have the issue Of non clering FCm and of and IIB but thats another topic
btw, you can go to the CFTC web page and find the financials on all of the FCM's and IB's and determine how much is firm capital and cust segregated capital...Good thing to know before you park ur money in some unknown destination...