Indeed by most measures the 6mm is ballistically superior (but barely) to the .243, but the .243 won the popularity contest between the two and the 6mm is becoming (if it isn't already) a reloader's round.
P.S. The .270 win and .270 WSM are long range rounds, in the hunting context, not mid-range rounds.... in my book anyway. They should be eliminated from the poll, as should .25-06, which is more of a long range round as well.
FF-
I think he meant medium bore cartridge. At least when I read it, that was the only way the poll made sense. As if most of these polls amke any sense.