well a 04 would be worth more then a 97 just by its age. im not 100% sure but that supercharged emblem on the trunk lid, im thinking that started in 2000.

as far as the regal vs the gp, it rides about the same, but is more comfy inside for sure, seats are way better. vs a 97-03 gp anyways, ive yet to sit in or drive a 04+

in the end its the same car with different body on it. same maint issues, lim, hubs, and so on.