I don't think there is any measureable difference in processing speed between Logix platform CPU's running the same code. The key difference is going to be in the sheer size of the program, number of tasks, communications connections and CPU memory. The attached document is from the inside cover of the FlexLogix Selection Guide (1794-SG001) and contrasts features of all Logix CPU's. Pros/cons: given the 2 choices you listed, it would simply come down to price (CompactLogix will be lower cost) and if motion control beyond stepper control is required (both platforms will do stepper control. If servo control is required, you will need the new CompactLogix 1768-L43 CPU, power supply, comms modules, and the traditional 1769 Compact I/O.