The original is good, but the character we follow around isn't terribly engaging and the whole reason for him being there (and how it all concludes) is hard to take. The opening, Lancaster, Gardner, Siodmaks' direction and look for the film, along with the flashback structure are ultimately what makes it work and a classic.
I've always liked the '64 version but it's managed to really grow on me even more over the years. Like you said, Marvin's character is far more interesting to follow, though at the same time I do have a hard time believing a hitman would ultimately go to the lengths he does to find out why his victim welcomes death, even if he's going through some existential crisis. Still, I can overlook this while watching.
The performances are all strong and they've all really grown on me as well. I know Cassavetes did the film for the money but I still think he's really good in it and I completely buy his character.
The scene where he realizes Dickinson has completely screwed him over (after she says something like "make it quick" to Reagan) is heartbreaking from his point of view.
Dickinson and Marvin are both damn good, and Marvin just steals every scene he is in, as does Dickinson, and when the two share the screen together it's easily one of the most intense moments in the film. But it's Reagan that always surprises me. From my perspective it is hard to take him seriously since all I really know him as is a former President and I had only seen his films after the fact: because I grew up in the 80s he's a politician first and an actor second. Also, how he has always had the same look, even in this film, makes it even harder. But he plays a really good heavy here, and I found him quite menacing.
His final moments, when he realizes the end has come and he has been screwed over by Dickinson as well, is especially effective.
And the ending is far more effective in the '64 version I feel:
Basically ends in a blood bath and I love Marvin's final line: "Lady, I don't have the time." Much more engaging than the ending they even make fun of in the special features of the disc, where the "hero" saves the insurance company a fraction of a cent on every policy they have.
It can have a low budget
The ending of the original feels tacked on and actually makes you forget everything that came before. Maybe it's a mix of growing up on pulpy films from the 50's, 60's, and 70's and my somewhat cynical nature, but the ending to the '64 version is far more memorable. I actually forgot the ending of the original and was only reminded of it when I revisited the Blu-ray here.