Take the train!!!!
I can't stress this enough. A taxi will be longer and cost 3 times as much. The train is about 7-8 euros to go to La Defense. You can buy the tickets in the airport and the train station is in the airport. The hotel is right next to the "La Defense" stop, so it is very convenient. You make one change at les halles like another poster said.
I lived in Paris for 6 months, and took this train a dozen times without a problem. In my opinion it is the best way to go.
Just my 2 cents
Cheers
Timo