The empeg and the alpine deck would most likely sound equally good.

The PC would sound inferior because the output stage of most consumer-level sound cards isn't as good in terms of noise floor and frequency response. Plus, in-car computer installations where you jerry-rig a desktop PC into a DC/car environment are going to be tricky to get right in terms of noise rejection.

Of course, with the alpine deck, you'd have to swap discs, you wouldn't get all the music organization features of the empeg, etc.