Originally Posted by
LukeSkywaiter
It's worth noting that you can now generally buy an actual ticket for less than the cost of a buddy pass roundtrip internationally.
That may be the case on CO, but it's not always the case on other carriers.
As for the OP's query, when I was doing more extensive CO flying (back in the 50% EQM days), my expensive tickets always went to NW. The upgrades were more reliable on NW, and it was 100% EQM regardless on the higher fares. CO's policies of that era simply ensured I flew CO on the cheap stuff, and flew NW on the expensive stuff.