From a capacity point of view, the +12v amps is the most important metric. That is what the most used components depend on, particularly the graphics cards.
Cheap psu's will advertise high wattage, but it is implemented on other, lower 3.3 and 5.0 voltages which are lightly used.
A quality psu will deliver it's advertised power continuously, not just at peak.
A quality psu will deliver that power at 50c, the temperature inside a case. not at 20c, room temperature.
The best psu's will have higher efficiencies, over 90% for gold rated units 80% for bronze, I think. More efficient units lets the fans turn at lower speeds, making them quieter. Cheaper units may be 70% efficient.
My short list of quality psu's will include Seasonic, Antec, Corsair, xfx, and PC P&C. You can safely buy any modern psu from them.
In the absence of any information, the better psu will weigh more as a rule, but nobody advertises the weight.
A respected review site is jonnyguru.com http://www.jonnyguru.com/index.php