If it were me, for only 5.5€ difference between them, I'd opt for Les Cars Air France to avoid having to schlep heavy bags onto the bus. The Roissybus is fine if you're traveling light, but basically it's a regular city bus with a couple of luggage rack spaces added. When boarding it I've usually had to navigate around people standing and/or leaving their bags in the aisles--not easy to do if you're encumbered with luggage. The Air France bus is a touring coach-style bus, where they load your bags underneath and you get to enjoy a nice cushy seat for your ride (and it has air conditioning, though that won't be an issue this weekend!).
+1 for les cars air france, however note the pitch is really small...