It depends on the car. Ultimately, the power limit is set by the fused source in your car. If your 12V hook up will support the higher current, get the beefy one because you'll grow into it. The minimum you should get will depend on what you are trying to drive.
What is the peak power draw of your laptop charger? Get nothing smaller than that. If you supply at least the charger's peak draw, anything more will not charge your batteries faster. But it's nice to have reserve capacity.