FCM is the member who clears your trade
IBs act as retail sales force for FCM.
IB's must put up bond money to cover any customer losses which can't be recovered from the customer. Many IBs do not have the capital to put up this bond (I forget offhand how much it is) so the FCM will guarantee the IB.
In my opinion, it is an error to say one is better than the other, for reasons too detailed to go into.
CPO is a disclosure that the firm is authorized to trade pooled money.
I guess you already know the abbrev
FCM Futures Commission Merchant
IB Introducing Broker
CPO Commodity Pool Operator