Agree with scimowser on these two points. Chuck will be a lot harder to take down in the cage than he is in a ring. He is a notorious "wallwalker."
I noticed that, too. Chuck has gotten really good at getting back to his feet once he's been taken down. Another point to consider is that the UFC has been standing the fighters up more and more quickly, also favoring Chuck.
If I had to bet, I'd go with Chuck, but I still give Jackson a good shot, if he's motivated. He has definitely faced the superior competition. It's not even close.
And people here make it seem as though he's old. He's 28. If anyone's skills will decline by the time they fight, it's Chuck's.