I've thought about this a bit. I prefer Robbo, I really do. In terms of consistency I can't think of a better player for Manly over the past three years than Robbo - I'd say he's really only had two or three games where I've gone "f*ck, ordinary game Robbo!". The one against Penny in the rain when he was fill-in fullback and was stepped by Preston and then Wesser to look very ordinary springs to mind. But 99% of the time he is the model player - no errors, great finisher, reads the play well and very good under the high ball. He's also not as old as many people think. Only 25. Unless he's asking for crazy money (remember, he is still "just" a winger) then I'd be stunned if we didn't retain him.
On the other hand Williams is going great guns and I love how hard he runs the ball - very effective on kick returns and a great boost for the forwards (although Robbo also runs the ball like a prop on kick returns). There are a few concerns over his ability to read in defence, but front on he is a good tackler - remember this was his first season in the top grade after a number of years where he did his knee/various other parts of his busted body. Look how much he improved this year as he had an injury free year. I can see similar improvement in him in attack and defence if he has another couple of good years with the body. I see Williams ending up in the centres eventually, whether it be at Manly or somewhere else. Also, and this may be harsh, but I can see him following the money a bit more than Robbo.
They're both excellent blokes and I hope they both stay, but in the end they are just wingers (at this stage) - if either of them prices themselves out of Manly's range, then cut them loose. We've got Bani on a healthy contract for another 2 years I think, so we probably do need to retain one of them. I'd be happy with either, but I think we'd get Robbo cheaper and we all know what we can expect from him every week.