Having done two day trips from Paris to DLP now, I can say 100% that the train on your own is the way to go.
The RER is quite easy. Take the Red Line to Marne-La Vallee Chessy (I think that's spelled right). It's the last stop on one of the two east-bound lines. It's about 30 minutes from the center of Paris, and costs 6.50 euro, I believe. With the RER, you can leave Paris and leave Disney whenever you want. The RER runs late, and is very safe. We left the park after closing time both times we visited, which was around midnight. At least 95% of the passengers coming back to Paris had been to Disney that day, and had the shopping bags to prove it!
With the bus tours, the price gets jacked up significantly, plus you arrive at the park after opening time and leave before closing time. Plus, even these tours that pick you up at your hotel often just take you to their central location and then make you switch to another bus, which can be frustrating.
We once bought our park ticket in advance at our hotel, and once just waited to get it at the park. If you buy it at your hotel, it's just a voucher, and you have to stop to get the real thing at the ticket counter anyway, so we preferred just buying the ticket at the park.
Hope this helps!