Clark had a better pro career, but only due to longevity. And that's not a knock against him either. He was a great TE for a high powered offense. Sanders was a truly elite player that very few ever reach the status of though. Sanders took a Colts defense that was Swiss cheese and turned them into an elite unit seemingly overnight when he came back from his knee injury in the 06/07 season. The Colts went on the beat the Bears in the Super Bowl and the vast majority of people agree it would not have happened without Bob. If Clark had missed 12 games and came back for the playoffs, his impact may have been felt, but it wouldn't have taken their offense from bad to elite the moment he stepped on the field.
I think I'm going to have to agree with
@fredjr82 on this one. Sanders did only have a few great seasons, but he's also the reason Manning won a Super Bowl with the Colts. Clark can't put that on his resume.