My parents have done coach on both CO and KLM on this route several times, and I think their preference is KL744 > CO > KL763.

CO has IFE in each 767 coach seat while KL doesn't on their 744/763s. But on KL's 744, they like those two-by-two seats in the very front Y cabin that they'll take them over anything else.

They also find the food a little better on KLM.

Yes, you can get seat assignment to the seats near the front of the cabin on CO, but don't know if you can do it online. Bulkhead and exit rows for Platinum only.