I can't say much on tablets (I've never used one nor really want one). But, I do browse the ipad and android front end threads that pop up once in a while. From what I've read, the tablet os's aren't that viable in the car compared to a pc frontend designed for the bigger buttons and whatnot, but they are incredibly stable and functional if you're not into tinkering with hardware/software.
PC's are the way for me and plenty of others here as well. You have control over what hardware, software, and any future upgrades you want to do. They are incredibly versatile if you are into it, and if done right, can be tons more powerful than a tablet, in functional software and hardware.
Ultimately, it's up to you and what's right for you and can fit your budget. PC's can run a lot for the car enviroment, with sunlight readable touch screens running 500+ alone if you can't stand sun glare compared to the normal non-transflectives still running average around 150ish.
What do you desire besides music and gps?