The KC-10A has certainly been popular with crews and it's combination capability is far greater than the KC-135R. I remember when there was talk of making a tanker based on the MD-11 but it never happened. On thing for sure, the MD-10 conversions are extremely popular with cargo operators. My question is, how many airframes are left available for rebuild if it became an option to go with a new MD-10 Tanker?
I've studied the KC-767 specs frontward and backwards and it actually comes in lower than the KC-135R number wise. It's major drawback is that it's airframe size and engine compatibility limit how much thrust it has with two engines and thus how much usable fuel & cargo it can carry. The KC-767 is closer to the 135R than it is to the KC-10A and A330 MRTT. The Airbus is bigger and has more powerful engines than the KC-767 which allows greater usable fuel and cargo loads. Still, the 3 engined KC-10 has the extra power and room for a huge payload and usable fuel. I lean towards the Airbus over the KC-767 any day but if the choice was open, I think the MD-10 upgrade into a KC airframe would be a better choice overall.
Some specs:
Bearing in mind that the weights listed are the maximums and the max fuel and cargo weights would not be carried at the same time without exceeding MTOW ratings for each aircraft. The numbers reflect the maximum for each typical configuration.
KC-135R
Max Fuel: 203,290 lbs
Max Payload: 83,000 lbs
KC-767
Max Fuel: 202,000 lbs
Max Payload: 77,000lbs
A330MRTT
Max Fuel: 245,000 lbs
Max Payload: 99 000 lbs
KC-10A
Maximum Useable Fuel: 342,000 Pounds(Max takeoff weight: 590,000 lbs)
Max Payload 170,000 lbs