Pontoise is a LOOONG way from town! I whent on a Bank Holliday and it was, to cut a long story short, £80 in and £60 back!!! The taxi's charge extra for an out of town pick up and big loading for Sunday/Bank Holliday!
Now i prefer Tousouss, 20 mins on the train which is not far from the airport. Also if its clear you route down the west side of Paris. There is also a Campanile hotel on site incase of enforced layovers!