1. When you say do you use MCUs or CMOS ICs, you are aware that MCUs can be CMOS ICs themselves? Are you really asking whether you should use a MCU or discrete components?
If that is what you are asking you should probably use an MCU unless your robot is to have really simple behavior. And if you are using IR or RF it will be way WAY easier and better to use an MCU instead of hardwire the circuit. That way, you can much better shape and mold the remote commands on your robot. Also...much easier to debug. If you hardwired a command wrong, that circuit is pretty much history and you will have to redo it. Plus, wiring so many commands up would take you lots of room unless you had the equipment do make it small (for example, the equipment used to make an MCU).
2. If its that small, I would go with IR. Being that small, it probably doesnt go very far and you dont need to send messages through walls. RF is also more expensive and to my knowledge considerably bigger. Even the MaxStream Zigbee module is quite a bit bigger than an IR receiver.