I'm more convinced now, but still not %100 sure. Have you tried yet using magnified live view? And if not is there a reason for that? The other question is whether it's on a tripod with self timer or remote? Shake really does matter a lot, especially at these focal lengths.
BTW I'm not implying you're doing something wrong, I just know that with long lenses, there is a whole ton to be gained by improving and then refining technique. The people who post amazing shots with the same lens as others who are frustrated with results from their first supertelephoto; I tend to have a hard time thinking it's variance between the lens copies, rather than variance between the photographers' skills.
One place where these shots clearly show the advantage going to the mirror, though, is the purple fringing. Check out those branches in the foreground with snow behind them!
I hope you're having fun doing these tests. Keep at it (plus real world shooting of course), and I bet your shots from both lenses will continually improve