The shop guy that says 12000k for your HIDs should be fired, gutted, then drawn and quartered.
Get 3000k.
And the only "real" way to really gauge how much you can see as compared to the other is to sit down in the car and look down the road. Looking at them 3rd person is not a good way to judge output. Firstly, if the G35 has HIDs equipped, the Civics will look MUCH brighter, since the headlight reflectors will likely throw the light haphazardly everywhere though mostly in a beam pattern.
The higher you get on the Kelvin scale, the less light output your eye is able to see. I mean, if we could see more with 12000k, we'd be bordering on UV vision.

Our eyes are "adjusted" to see better at noon-day type light. Which is about 5400k or something like that. So, accordingly, we'd see better with either 4300k or 6000k.