I would think a younger, cheaper back has more value in a trade. Jackson was paid 2 mil last year and Marshawn made 600k. I am assuming they both make about the same in 2010. Lynch may have underperformed for us and definitely has character issues, but he can still be a feature back. At that price, someone like Tampa or Seattle (where Lynch could be reunited with Cal teammate Justin Forsett) may very well be interested.
Jackson fits perfectly in the complementary veteran role, and can still be a huge asset at the same time. He is a tough runner, catches passes out of the backfield, was team MVP last season, is a locker room leader and is known as an overall class act . None of that can be said for Lynch, who hasn't seemed to have matured out of the PS2/Lil John portion of his life.
The more I read about CJ Spiller, the more I love the pick. But, if Fred Jackson is the odd man out of this running back log jam, I will be highly disappointed with this franchise.