had it been a single point HID kit then going for the used one would have slightly been a better option as only ballasts are more likely to be abused to an extent (well HIDs are made to be run for longer periods of time and not for sporadic ON-OFF in a shorter period of time)...used dual point kits have basically two drawbacks as paindoos sparingly keep flashing the high beams more than a million times a second so that usually has a detrimental effect on the small motor that makes the bulb elevate + the fore mentioned effect of ballast abuse...
i say, go for the new one...