Forget about either server OS. Aside from the cost factor (you are using a licensed copy, right?), they don't bring any advantage in terms of capability, and it's unlikely that they will offer the hardware and software compatability you will need to get the most out of your CarPC.
XP will provide the best compatibility with current hardware and software, and depending on your overall configuration, may offer better performance than Win7. Over the long term, Win7 may be a better choice as new hardware and software becomes available that takes better advantage of its capabilities as compared to XP.
Short term: XP. Long haul: Win7