I don't claim to know a lot about CD players, but one thing that I listen for is the size of the soundstage that each player presents. Portables have a smaller soundstage or stereo image. Other things that I look for clarity and fullness of sound, with good balance. Depending on the player, you'll get a different presentation, so sometimes you might prefer the sound of a portable if it gives the presentation that you like. I'd agree that for the most part you'll get better sound from a nonportable unit, but I have a sony one that sounds like crap (common bloated, muddy bass and no highs) that is inferior to any of mys portables.
I would sure hope that the DCM-370 beats the SL-CT570, because from what I've read lately, the Panasonics aren't really as good as they are claimed to be when compared to portables a year or two older. Lately I've wanted a ne portable, but the two older ones that I've got sound pretty good so I'm reluctant to get a 570.