The only option to fly in cruise maintaining altitude is ALT HOLD or CWS, VNAV ALT requires an FMS.
Yip, three options. Boeing recommend CWS in the 737-800 if the turb is severe. Apart from that , any of the three is fine as far as I can see.
What I wonder about is what different servo inputs come from the different modes, in order to smoothen the ride.
I imagine that once the turbulence is severe, it is not about smoothing the ride, it is about minimising the stress on the airframe. Correct me if I'm wrong.
In VNAV PATH the servo inputs are made to move the elevator to return the aircraft to the selected FL. The same for ALT HLD. In doing this there is stress on the airframe/flight controls. In CWS the servos would be holding attitude which would require less input and therefore less stress than the other two scenarios.
The above is just how I imagine it, I would be very happy to have someone with more knowledge correct me.
Cheers