Cosby was more accessible, he's permeated more of the culture, and he did it all without cussing.
Carlin got famous cussing. I know there's more to him than that, but... eh? He's funny when he's just observing and digging really deep, but he didn't have as good a "flow" as Cosby did. But some of the stuff he did when he was an old man was pretty gold. When he wasn't just flat out insulting those who didn't agree with him. And all the Todds.
I know a lot of people think Cosby is boring, but he wasn't so much a stand up comedian as far as today's standards go. He told a bunch of stories, slowly, weaving humor in slowly. Watch Bill Cosby Himself and watch how he does two bits that are quicker and punchier, then goes into the rest of the stories and just builds and builds and builds on them. You get some laughs in the beginning, but then just a giggle here and there, a good laugh when the smaller stories wrapped within the narrative, but by the end? Everything he says is satisfyingly hilarious, but those jokes took time to get to and build. Carlin couldn't do that. He just got to the point, which has its place, but it is far more rewarding to build up slow.