I won't comment on Fitz because I've admitted I think he'll be better.
Clayton will take time but he has the tools of a stud WR, he needs to work on his speed and strength but those are improvable parts of his game. He has great character and give him a few years and he'll be great.
Roy is the more talented receiver - few would question that. He's so talented that no one cared in Texas about the little things that he sucked at. He's the opposite of Clayton character-wise though...bad attitude, inconsistent, sometimes he seems like a pansy out there. I don't see why his injury problems won't continue as well. He could save himself and become Moss like (Better than clayton), but the Lions took a gamble in my opinion because he's a great bust candidate. I see by your sig you're a Longhorn fan so I doubt you'll believe a word I'll say
Clayton's first year or two in the pros will be spent working on his speed mostly, and strength. He has good playmaking talent that the Bucs are going to tap into to go along with the consistency that he already has shown. Clayton played just about every position for LSU and needs some polishing, but once polished he'll be a gem. And just as important, he's a considerably lower inury/bust risk than Roy Williams.