Ok... in my limited non-extensive non-conclusive non-audiophile subjective "testing"upon which I am now forming opinion...
I've tried a toshiba satellite, sony vaio, mac, dell something, acer aspire, compaq something. Except for the mac, all had some noise (albeit some only barely). Unplugging the computers generally reduced the noise significantly or entirely. The mac had virtually no noise plugged in or not. My Toshiba was the worst; even unplugged, there was a some noise. Yes, I did muck around trying to remove ground loops. I also have a ground loop isolator, which does a reasonable job of cutting out the noise, though not completely.
Testing for noise on the various laptops is simply plugging the computer in without playing music, crank up the speakers and listen for any buzzing or hum. Of course, I'll also just turn up the dial without having the computer plugged in, to see if there's any hum inherent to the speakers.
On my own computer, I've played out through my desktop speakers, my living room speakers, a whole host of portable speakers, through a pro setup with enough power for a rock show in a gymnasium, and on live radio. The more powerful the speakers I was using, the more noticeable the noise was (plugged in or otherwise).
In terms of listening to various things side by side, I did run my computer's onboard, whatever usb/firewire output I decided to try, ipod shuffle, and sansa clip through a mixer and out a single set of speakers. Played the same song on each (except the shuffle) and compared the various channels to each other. Equalizers flat on all the devices and mixer, device volumes set to ~90%. Yes I know that not all devices reproduce the frequency range equally. Whatever.
In terms of my rating of what sounds "better", I'm defining that by how much "noise" a device is generating (measurable), and how "clear" the sound is (subjective). Playing some songs off an ipod or my clip, I can hear some background instruments that I couldn't notice when playing through my onboard (or weren't as well defined). Vocal separation from the instruments sounded better. Tones sounded more "crisp" for lack of a better word.