I too agree that either integrated is a great little unit, and you would very likely be happy either way. However, were I making that choice with that set of speakers, I'd choose the NAD.
Personally, I think the character of the sound of the NAD will work better with the tube-loaded metal dome tweeter in the DM600 S3's. The NAD will very slightly emphasize the upper bass and midrange, and smooth the highest registers. It adds just a slight degree of warmth to the sound of the music. That is a desirable combination to my ears.
The Rotel is a great little unit, and if you want energy and drive, then it might be more to your taste. It would also probably work better if you're into thumpin', screamin' rock or techno. To me the Rotel sounds like it has more emphasis in the higher frequencies. This is probably not actually any real emphasis, it's motr likely that it's really more accurate and flatter... but as it does not bump up the frequencies in the upper bass and mid's like the NAD, the highs come across as more "up front"... if you follow.
That's my two cents worth, anyway. Make your choice based upon your musical and personal preferences.