I, too, did not know there was a shuttle/train, since I always arrive in Termnal 2, and from there it's a bus, or a walk - I've done both.

I take the Air France bus, if my guess is that traffic is not too bad (Sunday is always good), and the RER when its rainy, or because of the time of day, my expectation is that the roads are not "fluid". Of course how much luggage I have, or my hotel location also influence the decision.