I would choose the ibiza mp3 player (especially if you have an adapter to charge it). I don't have any personal experience with the particular device, but it was made to play audio, so I assume it will sound good enough. Also, it is small and should use low power, which I feel is a plus in a car.
Laptop would be my second choice. But it's larger and having to use the trackpad/mousestick in a car can be difficult while driving.
I'm guessing the phone audio probably isn't better than the mp3 player. And I'm not sure how to power the soundbridge in a car.