Phil, you've hit the nail right on the head with everything you've said there. The much quoted 10:1 kill ratio is a bit of an over simplication of things. If it related to just Sabre vs MiG with NK or Chinese pilots only, then it would be fairly accurate. Sabre vs MiG with Soviet pilots only is a different story as you've said. In recent years a few records have started to come to light which might slightly favour the Soviets if those records are correct.
Another factor is the two side's different criteria for claiming kills. I'm only referring to to the Soviets here and not North koreans or Chinese. The U.S. mainly relied on gun camera footage, with the presence of smoke from a MiG generally being accepted as a kill. This was good for the U.S. pilots and their units and good for Uncle Sam's propaganda machine. Overstating kills has been around for a lot longer than the Korean War. It looks good in the news back home and helps build public support. All sides like to feel like they are winning, it's good for morale.
The problem is, quite a few of those smoking MiGs made it back to their bases. The MiG-15 is very hard to kill with .50 cal. bullets and has a fairly good fire suppression system, so a lot of claimed kills by camera footage were able to dive away and eventually bring the engine fire under control and limp back to base. So there has been cases of the same MiG being claimed twice (usually they did get it on the second claim).
The other side of the story is a bit different. For the Soviets, gun camera footage alone was not enough evidence of a kill. Usually reports from ground troops and preferably physical evidence of wreckage found on the ground was required as well. Also, Soviet pilots were paid a combat bonus of 1500 Roubles per kill so the authorities were naturally cautious when approving kills. So in the end, there's a possibility that U.S. kills were slightly overclaimed and Soviet kills underclaimed.
The question of which machine was better is always debatable. Comparing the MiG-15 to the Sabre, the Sabre wins on most counts. Comparing the MG-15bis to the early Sabre, the bis had a lot of advantages - higher ceiling, higher rate of climb, speed etc., but by the time the F-86F was out later in the war, in my opinion it was the better of the two.
The MiG had better survivability, although the Sabre could handle a bit of 23mm.. The problem was, a couple of hits from the 37mm. and it was Goodnight Irene.
I guess it still all comes back to what Yeager said, it's the skill of the pilot. Even with the heavy armament, the slow rate of fire of the MiG's cannon meant they were only really effective in the hands of a skilled pilot.
I think with all the MiG-15's flight characteristics, you would have to be skilled pilot to fly them well in combat.