I also have both these players and spent a fair bit of time doing a/b listening tests (I've somehow ended up with some duplicate CDs and had a spare set of interconnects to use). I found with the Marantz that the vocals seemed more forward and fuller, like as if the singer was closer to you. The Marantz also seemed to be a bit more bloated in the bass. The Arcam seemed the more detailed of the two and had a wider, more open sound. I also tried the CD players using my various amps, speakers and cables, and the results were pretty consistent
Up until the Arcam started playing up I ended up swapping between the two, depending on my mood. Extra detail and openess aside, I think the Marantz is a more emotionally involving listen - it has a better foot tap factor. You could say the Arcam is the more audiophile, while the Marantz is the more musical. This maybe why they are still so popular today and are considered a classic.
As an aside, I noticed similar comparative traits when I compared the Marantz to the MF Trivista player I had.