The stuff I'm reading on Bush states, for example, "Would be best playing closer to 230 pounds, but his inability to control his weight and durability issues - perhaps closely related - have kept him from realizing his potential. Needs to adhere to a strict nutrition regimen, show more discipline and get healthy." That's from Profootball Weekly 2007 Draft Guide. I'm not sure if Bush is a HIGH probability of being better (or as good as) JJ.
Lynch seems interesting. But I'll just add, that according to PFW, " .... Lynch has never shown he can be a featured back, and much like the Cowboys 2004 second-rounder Julius Jones, he could struggle staying healthy, even in a two-back system. A highlight tape could be deceiving."
That said, Lynch does appear to be a 3-down back, has had good production in college, etc. I guess if the coaching staff was definitely looking to upgrade JJ, the best realistic bet would be Lynch - though not a sure thing as an upgrade.
My preference is unless I get a 2nd rounder for JJ and I thought that Lynch was a clear upgrade (meaning I'll have to possibly use that 2nd rounder to trade up for Lynch), I'd rather just keep JJ this year.