The different models in the same line being different qualities, are typically linked in with who actually manufactures the unit. If you get a GX550 make sure its one of the new ones model RS-550-ACAA-D3
The quick way to tell them apart is that the new model is Bronze 80+ and the old unit is Standard 80+
Look for the bronze...
Here is a review of the new units little brother: http://www.hardwaresecrets.com/article/Cooler-Master-GX-450-W-Power-Supply-Review/1234
eta: one thing I don't like about the Jonnyguru reviews, are the Value and Aesthetics categories. Those categories can pull down the rating of a good unit, or boost the rating of a bad unit. Value is ephemeral, and aesthetics is in the eye of the beholder (plus a good portion of the time, its unseen)