There's a lot of factors that go into what makes a projector better. As far as what affects projector quality, there's the shape of the reflector bowl and how reflective the chrome is. There's the spacing between the bowl, the cutoff shield, and lens which affects cutoff sharpness and color, the spacing of the bulb in the projector, the clarity and shape of the lens. A clear lens will produce a sharper and more colorful cutoff. Changing the spacing between the cutoff shield and lens affects cutoff sharpness and color. There are many other factors that infulence the quality of a projector, but these are what I can think of off the top of my head.
The quality of the bulb and ballast makes a big difference too. An OEM bulb and ballast will outlast, and outshine a kit by miles and last YEARS longer. A kit bulb is only expected to last something like 200hrs before losing brightness and color shifting. An OEM bulb will last for thousands of hours.
As far as what makes a projector good? That's somewhat subjective. Some people want a very smooth beam, which often isn't as bright. Some projectors have a streaky beam, but have a much more intense light. The wider a projector is, the better obviously. Some people like having a sharp cutoff, which usually isn't very colorful. Some people like having a thick colorband. The higher quality the projector, the better it will focus the light from the bulb and put the maximum amount of light on the road.
Anything I missed?
I'm glad to see the sudden interest in retrofits on this site, it doesn't seem to be something that's very big in the GP world, while it's huge in the Cobalt world and in many hondas and such.
