To me it comes down to: Who would be the more effective starter next season Allen or Jackson? To me that answer is Jackson. He's younger, faster, better defensively and is close to equal offensively.
Jackson is no-where near as efficient an offensive player as Ray Allen is. He also is overrated as a defender.
I think he is a good player, but I would not take him and his contract over Ray. Ray, even with his age, is still a superior player to Jackson, and he will likely remain that way for a few more years, given his level of fitness, and his style of play.
If the C's can sign Ray to something like a 2 year, $30 million deal, or 3 year, $25 million deal, it would end up being a much better use of resources.
And this is not even mentioning the attitude problems that Jackson brings to the table.
Now, if the C's decide Ray is leaving (or he refuses to play for reasonable money), then I would much rather they go after a younger player to replace him. Making a run at someone like say, Josh Childress, would be my first choice.