Some of my thoughts on Wilt from another thread:
This could also be said to apply to Russell.
Wilt is more skilled than Howard, on both ends. Wilt could definitely compete and dominate against today's bigs.
The issue with Bill Russell is that he's undersized by today's standards. At 6'9" around 220 lbs, he'd be like Scottie Pippen in today's game, not really big enough to be a starting center normally.