RER vs. Metro
Answer to the RER question.
In my opinion the Metro itself is pretty safe. We were on the RER on return from Versailles and literally saw some guy grab a womans purse and run off the train as the doors were closing. Didn't get my opinion out of a travel book but saw it first hand about three weeks ago...
Anyway, with all of that said after a long Trans Atlantic flight I'd much rather have the convenience of a Taxi. Yes you can take the RER from the airport. But if you've ever taken the Metro you know that you have to walk up and down stairs, walk a long ways sometimes through stations to transfer, then walk down the street to the hotel. Do you really want to do that with your bags after a really long day????
The buses do get you closer but you've still got to take a Metro or bus once you get into the city.
Our taxi from CDG to the Hilton Arc de Triumph was 47 Euros. At most your difference is going to be 20 Euros depending on what mode you decide on. After making the long trip over there I am willing to pay an extra 30 bucks to be delivered privately, directly, and safely right to the front door of my hotel without the hassle of dragging my bags through the train stations and up and down all the stairs in the Metro stations.
To each is own but I think it's worth another $30.00 or so for the taxi.
Enjoy!