I was forced into the same decision this weekend as I had both Williams and Harrison and made a move for Mendenhall. I ended up dropping Harrison.
That's not to say I don't really like Harrison. The guy has always been solid spelling Lewis but as an every down back, his durability could be a question. And the Browns are horrid. Cleveland seems to systematically destroy their running backs by handing them the ball 30+ times a game and letting them run dead up the middle where all the hits hurt the most.
Williams to me is more valuable because Miami is a run first offense and despite Ronnie Brown being the guy you think of when the Miami running game comes into conversation, Williams doesn't slack for opportunities. Averaging 16 rushes and almost 3 catches for 103 all purpose yards and 1 TD per game.
That said, Lewis comes back from injury, he's likely back to getting the rock 30 times a game and Harrison becomes obsolete yet again in fantasy. Williams though, isn't going anywhere until the end of the season since he is talking retirement. He's the kind of guy who would want to leave the league with a statement made.
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