There seems to be some general consensus about these two guys:
They are the two best volleyers of the open tennis era.
It is difficult to say or gauge who is greater, but we see McEnroe's volley as being smooth and natural while Edberg has a more "textbook" volley, one that is ideal for teaching.
The following quotes from Internet discussion forums say it well:
"We teach the half or low volley as, get your head and eyes down closer to the plane of the ball by bending your knees and trying to keep your racquet head above the wrist, hell, Mac remains almost straight up, drops his racquet head straight down so his hand is at 12 o'clock and the racquet head almost at 6 o'clock, arm almost straight, no back swing, and a swat with a follow thru, and yet it goes where he wants it to. You can't teach that nor try to teach that, the timing required to pull it off time and again, well you need Mac's special gift and yes it is beautiful to watch."
"McEnroe's insight and physical application of thought was gifted beyond any S&Ver i've ever witnessed. Edberg was great, but his was a more "technical" form of the volley, where one would have an easier time learning it than McEnroe's which (aside from what was noted above) was genuine tennis/sports artistry. Few in sports ever reach the artist level, no matter the accomplishments."
True, McEnroe is widely regarded as a serve-as-volley artist indeed. I wonder if it has something to do with his being a south paw.
Another common thing about McEnroe and Edberg is that they are both excellent doubles players. They are the only men to have been ranked No.1 in both singles and doubles since the ATP computer ranking began.
In any case, there is one thing for which there is definitely no comparison between McEnroe and Edberg. Sportsmanship. During McEnroe's career, his behaviour was as outrageous as his talent, earning him the image of the bad boy of tennis, and he was quite fittingly branded as the "Super Brat". Edberg, on the other hand, was the ultimate gentleman, having won the ATP Sportsmanship Award for five times. He was so honoured that the ATP renamed the award the "Stefan Edberg Sportsmanship Award" in 1996.