Mobile processor would not only draw less power but run cooler but I've heard of people using desktop processors with no issue.
You know I was also wondering whether to get the mobile processor. But let's suppose this way - most of my time, I am using the car pc would probably be around 3-4 hours a day. Would a mobile processor make a difference? To my knowledge, the mobile processor would just make consuming power more efficient. Important difference for a laptop battery but wouldnt make any difference for a car battery. Dont you think so?