Ewing played on a far lesser team than Reed. I think Ewing could've been Bill Russell if he played on a team as stacked as Reed's. If he had all star perimeter players around him he could've been an eliminator on defense and scored with his athleticsm and in the low post when needed. From watching his old tape and Georgetown, you could see that his instincts/athleticsm were unreal. He was on another level defensively. That said without seeing everyone minute of Russell's peak the way that I watched Ewings, I can't comment on who was the better player. I just know one team had a bunch of hall of famers and Ewing only played with guys earned 1 or 2 all star appearances after emerging from complete obscurity.
Ewing had a great offensive game in the peak of his career. He was not "stretch 4" or whatever he was labeled as earlier on this thread. Ewing could hit the 18-20 footer with regularity and that was given to him by NBA centers who didn't want to step out to the perimeter. This made him a more dangerous offensive player. But Ewing primarily operated on the low block with his back to the basket and regularly went to the drop step in the lane, or used to the turnaround from 12 feet.
Given he was 90% of the Knicks offense for most of his career, that made it very difficult for him to focus the kind of energy on D needed to be a Bill Russell type. Still the guy averaged over 2 blocks per game every season of his career till his minutes declined. I think some posters on this board only maybe saw him after '97 or so when he became more of a jumpshooter than his early career. Later in his career his knee problems and other injuries zapped some of his ability and then we also finally added players like LJ, Houston and later Spree who had offensive ability, and then finally Camby who added size to pair with him.
The biggest flaw with Ewing's game is he was a bad ballhandler and he wasn't a great passer out of the post. He didn't pass of the double well and couldn't put the ball on the floor much. The best Center in NBA history at those two skills, who also had the best combo of power and athleticism, was clearly Shaquille O'Neal. I think outside of Jordan, Shaq was by far the most difficult player to guard in NBA history. Shaq couldn't step out and hit a J, but no one could keep up with his power and speed. And when you tried to double him, it was worse, because he found open 3 point shooters. He was really an unstoppable force before he broke down a bit physically.