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?
So to quote Captain Planet " the choice is yours"
Have you given any thought as to where you going to locate your PC. If you want it in-between the seats you may be able to put the PC in the storage compartment between the rears seats(the vertical one that faces the trunk) if heat isn't too much of an issue where you live.