Even though I see a homer trend here. I must vote for the recently dearly departed Mr Webster.
Dwight Stephenson was a great center for a few years. But the key was for only a few years. Stephenson only was a full-time starter for less than six years. Webster was a starter for fourteen years.
Webster was good enough to force out a very good center, Ray Mansfield, by his second year, where Stephenson had not rooted out a mediocre center in Mark Dennard until his third year(you might be surprised that Dennard was the center in the legendary 1981 double overtime playoff game against San Diego and Stephenson played only on special teams).
Also the guards Webster played with were much weaker players than the guards Stephenson played with. Stephenson played with Bob Keuchenberg, Ed Neuman, and Roy Foster, all good guards who played in Pro Bowls, while Webster never played with a Pro Bowl quality guard. Yet the Dolphin and Steeler middle of the lines were rated nearly equally. Webster carried a bigger load.
Also sentimentally I must pick Webster. I shared the same heigth, played the same posistion, had the same first name, and wore the same number as he did. Webster was the only center wearing #52 in 1976 when I was given that number as a high school center, therefore he was the player I looked up to.