The biggest difference between the cheap kits and nicer kits is the ballast and the bulbs.
The cheaper kits run a DC ballast while nicer kits like the Morimoto kits run an AC ballast.
Most kits are 35 watts while some claim to be 55 watts. The cheaper kits pump 55 watts into a 35 watt bulb where as the nicer kits will come with a 55 watt bulb.
Slim ballast and DC vs. AC vs. square waveform design ....
I have 2 DDM kits and have yet to try them but the online reviews on them are hit and miss since they come from a factory in China.