I think Watson has the least long term upside for a few reasons:
1) He's not exactly young himself and was fair, but not great.
2) Historically, Rudi is fairly durable.
3) The Bengals have a bye this week - one less opportunity for Watson and another week for Rudi to get healthy.
IMHO, if you have Watson and didn't start him this past week, you may have already missed his best week (barring a new injury to Rudi)
Inicidentally, I am a Rudi owner and could have WW Watson, but chose not to. Instead I started Jamal Lewis and also have D. Ward, L. White, C. Taylor, W. Dunn and B. Leaonard - who will rotate in until Rudi gets back - which will likely be week 6.