If all you want to do is run a laptop , you will be better off getting a 'auto-air' laptop power supply - then you only have one conversion happing instead of two.
When I travel I take the 'Auto-Air' power supply for my dell - it takes AC power AND DC from car (and aircraft) power sockets.
I guess if you are driving you don't care about power effciency, just a few more watts of power sucked out of your petrol engine - but it you have reason to run with the engine off and just using the battery, every bit of extra effciency helps.
I work with guys who do need to run laptops for long periods with the engines off - and our work cars are fitted with larger batteries, alternators and hard-wired 1kw inverters....
And the 1kw inverter near melted it's leads when first installed. The thing is now connected to the batteries with what is effectively electric welder cable.