It's a tricky one because both versions have their merits. (Bearing in mind that Directors and Final are not vastly dissimilar).
I like the narration in the original because it makes it feel very film noir and I personally do think it adRAB something worthwhile. However the ending of both versions may possibly be the clincher for which version you prefer.
I believe that the Final Cut is more about polishing up Directors cut and getting it just about as perfect as the Director wanted.
If I were you I'd probably like to own both.
Possibly watch the original first, because that's what came first anyway, and then watch Final Cut.
Or maybe you'd like to watch Final Cut first because that was how the director wanted it to be like?
They're both very good either way so it's difficult to advise you one way or the other.