I've been shopping for HID headlights all across the web and car forums. I see debates about which id better: A/C systems or D/C systems. Any opinions? Also, it seems like DDM Tuning is the "favorite" for most drivers, but I've seen the negatives about them in relation to A/C & D/C systems. Any opinions on that? Thanks folks!

 

I don't know that I've every seen any HID kits specifically advertise A/C vs. D/C. I mean the car runs DC from the battery so I assume that's what all the kits are.

 

The HID lamp needs a high voltage to start, then a lower voltage to hold an arc. In any case, this requires a switching circuit to bump the voltage from 12V to thousands for startup and ~100V (depending on the gap) for running.
I don't think there's much benefit of one system over the other. AC can be seen to flicker if you use it with video systems - though it is a very high frequency so that may not be very realistic. A benefit of AC is that it erodes the lamp electrodes more evenly... that being said it should be noted that the lamp design would be different for the two styles of HID bulb.
I'm not sure if the stock system is AC or DC (not going to guess) - but you don't want to change since you'll have a hard time finding bulbs.