Its funny. The films are pretty much the same story, but the difference in 30 plus years of filmmaking is apparent.
Point Blank has very realistic, clear color imagery(thought the editing is New Wave European all over the place)
Payback is filmed in a weird "gray blue" that suggests a black and white film made in color.
Point Blank is fairly low key and realistic about the deaths of the various villains.
Payback goes for modern-day "tough humor" (as in Gibson saying he'll shoot a guy if the answer is "no" from his boss, and the answer is "no" and he shoots the guy.) (as in Carroll O'Connor's scene with two guys with suitcases turning into James Coburn's scene with two guys with suitcases -- in Payback, the suitcases becoming a joke(Gibson makes the two men hold the suitcases so they can't go for their guns; Coburn fires them on the spot.)
In certain ways, Payback is more action-packed and entertaining, and Mel Gibson does a good tough guy ACT.
But in Point Blank, Lee Marvin is, quite simply , a tough guy. He doesn't need all the scenes and schtick that Gibson needs to act tough, he IS tough.
But we'll never have a good comparison of Point Blank to Payback because...Payback was shot with one ending and then reshot with another and we'll never really know which story was better. Point Blank is a movie that was made with the story as intended.
I like both movies. I think Payback is actually more "fun" than Point Blank. But Point Blank is the real deal, a 60's art film noir classic with one of the real tough guys in American film.