We clear our business with ADMIS, and would be more than happy to assist you and discuss their approach to risk, seg funds, technology, and back-office support. Please PM me if you wish to schedule a call. I would be happy to help.
I really think the choice of FCM depends on what type of trading you are looking to do and your general trading needs. Like I read by my colleague RobertMorse mentioned, bigger is not always better as we witnessed with MFGlobal about 8 years ago or so.... We are an Independent Introducing Brokers working with 7 FCM and be happy to provide an opinion via PM based on your trading needs ( selling options? day trading? size of account? specific software? etc.)