Company type for Futures Trading
I guess for your type of business LLC would suffice, but you should contact an attorney licensed to practice in your state, because each state has its own peculiarities.
I presume that you are licensed commodities brokers, and you have appropriate securities counsel. You will likely need a specialized LLC operating agreement that has special language and a bunch of securities disclaimers, especially if you plan on selling interest in this LLC to others. Usually, the securities attorney should draft the operating agreement in this circumstance.