We've just done Les Cars Rouge and although it was nice, their stops are "mysterious" and hard to find! While L'Open Tour buses go through the loop at the Louvre, right where you can see them, Les Cars Rouge stop around the back of the museum, way up by the far corner, and don't even try to figure out which stop they're going to use at the Opera Garnier- could be the one that's marked, or it could be...where ever the driver feels like stopping.
The good thing about all of these is that you can get on and off where you'd like and use it as a method of getting from "here to there".
Enjoy your trip!