iDTGV is cheaper and as said above the iDTGV trains are just additional cars with a single fixed ticket price on certain regular TGV trains. iDTGV tickets are only available online for printing at home and the customer service and booking systems are separate from the TGV. You should not be surprised if you use an American credit card and it is declined. The iDTGV booking system is even more finicky than the SNCF system when it comes to taking US credit cards. If you take the iDTGV, your ticket is usually scanned as you board the train and there are no more ticket checks while en route. Also, while the iDTGV cars are attached to the TGV cars, you cannot walk between the iDTGV cars and TGV cars. The iDTGV cars are generally a little older than the TGV cars. Go for Zen cars unless you are one of those loud people who should really be in the Zap cars. I've heard that there are planned "events" in the Zap cars to encourage social interaction -- I have no idea if that is actually true or just an exaggeration. TGV Leisure/LOISIR is just a fare category. If all you see are LOISIR fares, that means that all the cheaper tickets such as PREMS are sold out. The good news though is that LOISIR fares are not the highest price point for tickets and they are exchangeable for free up until the day of or day before (I'm not sure) travel and can be exchanged for a small fee after the deadline but before scheduled travel time.