First off, I think Chad will be gone after this season. He's accomplished what he can since 2002 and is no longer the QB of the future. He's still good enough to at least compete for the starting job with the Jets, and with some other teams.
Despite his injury problems over the last few years, the Jets management has seemed satisfied with him enough to not pursue a major free agent or first round draft pick QB (such as Jay Cutler, a first year starter) to compete with him or replace him outright. He does have 2 wildcard playoff victories, though the Jet D played very well in both games.
The recipe for success for Chad is a solid running game with play-action and short passes and overall good game management. He's not a gunslinger who can rack up yards in a 4th quarter comeback and will fail in that capacity.
Kellen was drafted in the 2nd round of 2006 and was considered a developmental project. I think he was put into the starting job too early and Mangini is creating a controversy by bringing Chad, the likely favorite to start, into the mix. If I was Chad, I would walk without the starting job, so I expect him to make waves if he's not the starter. He's unlikely to accept a back-up/mentor role. But if Kellen wins the starting job in camp, it should be the indication that he's ready to go.
To me, it's nothing personal. The optimistic results could be:
1) Chad wins the job and keeps it throughout the season by staying healthy and playing classic Chadball (yes, dink n dunk). Kellen learns what Chad has to offer, mainly play-action and game-management skills. Jets have a winning record. Chad increases his free agent value for 2009. and signs with another team. Kellen starts in 09.
2) Kellen wins the job. This is a signal that he's as ready to go as he's going to be. There are some hiccups but he also brings his greater arm strength to the table, improving the Jets vertical attack and outside passing game. Jets have a winning record and Kellen is the incumbent franchise QB. Ainge is allowed to develop. Chad is traded early season for a mid-round pick.
The pessimistic outcome is:
3) Kellens wins the job in camp, Chad demands trade. But Kellen doesn't play well and it ends up being a long season with much free agent money spent and only 6 or 7 wins to show for it or they may turn to Ainge, a total unknown at this point.
My preference is to have Chad start for one last year and then to have Clemens and Ainge compete in 2009.