it all boils down to personal preference. With a minpc you have something that is relatively secured to the car and the expandability is not limited to the hardware. You're making something more integrated with the car, almost like an OEM installation.
With the UMPC you have the option of taking this hardware with you and using outside of a car setting. Only problem is the hassle of hooking it up and taking it out.
Decide which one will get the most use. With a built in system you can always upgrade and modify the system to what you need. With a UMPC you're limited to the predefined hardware specs of the system. But are you going to offset that by being able to utilize it a lot out of the car. Ask yourself these questions and you'll have your answer.