A Windows box is more feature packed than a tablet, but it has its disadvantages. The two biggest are startup/resume and cost. Cost-wise, an Android solution is better. My Nexus 7 cost $250. I'm sure you are aware of its features, but the key ones I wanted was MP3 and video play back, Navigation, ODBII and Pandora. I get all of these out of the box plus the cost of one application (Torque) to do my ODBII. Also, the start up time for my tablet is near instantaneous. With windows, I always had to wait before I could use the system. On top of that, I always had driver issues with the many USB devices I had to have attached to the PC. The tablet is a much easier and cleaner install. I run the output of the headphone jack to an outboard EQ and to my amplifiers. Some users are hosting their Audio over OTG to a USB DAC for near audiophile quality sound.
I agree that you get less with a tablet than you can with a PC. However, I feel the trade off is well worth it. Oh, there are solutions to the reverse camera issues. I just don't want to deal with bulky cameras or flaky capture devices that may or may not work over OTG Host. Go out to the Rootzwiki forum and do a search. You'll find a thread from Timur who has doe a lot to modify the kernel of Android tablets to work as carPCs. It is getting better everyday.