I've had both 594 mahogany, and the 594 with a flame maple, extremely subtle to almost no difference, the maple neck might possibly have slightly more clear string attack, but it could also depend upon what fretboard or what Rosewood piece you end up with, I went and played a custom 24 versus a CE 24, in guitar Center before I purchased the maple neck 594, I had a hard time telling the guitars apart. I really did like the CE 24 as well.
extremely pleased with my maple neck 594, I like a slightly brighter guitar and slightly more string attack, but I would guess you can get the same identical sound with either guitar with the slightest turn of a treble EQ knob. I think others have suggested the same thing whichever piece of wood is more pleasing to your eye is more important than the tonal differences.
here's my gorgeous neck