Since you asked about the Marantz PM5S2, I would tend to lean this way. This Marantz is part of their reference line and is in a whole different league than their AV gear. It just doesn't seem to be as well received here in the USA vs. other foreign markets. Unless you have heard and operated one. I have and am very impressed. Also it is a full featured integrated having tone controls, a quality phone section and home theater bypass feature. Plus at 41 lbs., its a pretty hefty piece.
The Naim is more minimalist for those who like this style and function. I am sure it is a very good amp - my preference however is clearly with the Marantz. I would encourage you to see, hear and operate both if you can before making a decision.