Janeh's absolutely right that there's just no way to predict how long it will take you to get your bag and get through customs. Last summer, I was out on the curb in 25 minutes, I felt like I had won the jackpot.
I agree that the later train is probably safer. As far as getting to Montparnasse, I would recommend the Air France bus. It leaves regularly from right outside the terminals (signage is fair, but I got information from someone at an information desk). It is non-stop to Gare de Lyon and Montparnasse and is pretty much hassle-free.
As far as navigating Montparnasse, I waited in the wrong line once or twice, but the clerk was able to help me and put me in the right one. I found that the clerks' English was of a much higher quality than my French. I don't remember the details too well, but I think I remember that if you reserved your train ticket online from the US, you will not be able to use one of the machines to pick it up but will need to stand in line. Also, there are lots of food stores right outside the station in case you need to get food for the start of the walk.