Making a rough approximation, all SX2 does is buffer the data from the peripheral and send it to the host over USB and vice versa. Unlike, the FX2LP, the SX2 has no processor on-chip to modify the data received from either the peripheral or the host before committing it to the destination. Hence, it is not possible to modify packets with the SX2. However, the FX2LP supports modification of packets before committing the packet to either the host or the peripheral. Hence if your application needs you to modify packets you before committing then you might want to consider the FX2LP.