The 448pg version-the first link provided-contains extra material written by other writers and editors plus some stuff from Dave Gibbons. This includes some character sketches early pencils and such. There is also an intro(i believe with dedications to his loved ones)written by Gibbons.
IF this is the edition I once owned with the same cover the intro is actually pretty cool. Gibbons gives some background on how the project came about and the umm lets call them special quirks Moore has when it comes to writing style. Also how the stress and demand the book required affected him during the project. An epic of a story where almost every page is 12 panels!? I am unaware of any work of the same caliber coming close to that kind of hand cramping gusto. I cannot remember if Alan Moore provided any extra introduction or epilogue in this ed.
The story and art are essentially the same in all editions it would seem. Most of them have the same page count of 448. The differences are really in bindings it seems. Link1-paperback w/intro&sketch,Link2(436 pages actually) along with the last link are the only odd ducks. The rest vary in weight and prize due to the quality of covers. Thickness,extra book case,slips,etc.
More pages equals newer with more ppl gushing over the book no doubt. The comments do not mention any difference in printing,paper,or color application. So all are most likely the newer remastered coloring.