In the past few years I've only travelled on the TGV, so I'm not sure how much help I can be.

On the TGV, second class is pretty comfortable and certainly more than adequate for the various journeys, which are generally no more than three hours or so (I took a five-hour trip between Paris and Biarritz in second class recently; no problem at all). During heavy travel season some passengers have to sit or stand in the baggage areas at the end of each car. But you generally need to reserve seats, so you probably won't end up with the baggage.

First class is surely more comfortable, with three seats across (one seat on one side of the aisle, two on the other), as opposed to four across. The seats are plusher and there's more legroom. But of course you pay a premium. While I definitely prefer 1st class (who wouldn't), it's simply too expensive for my family, and really not worth it if yu're trying to save centimes.

Either class, the TGV is a terrific train: comfortable, smooth and quiet -- and fast (up to 300 kmph/186 mph). It's my favorite mode of travel in France and to London (on the Eurostar, which is little different from the TGV; 1st class on the Eurostar is more luxurious: they serve you dinner and drinks. Otherwise the trains are almost identical.)

Bon voyages!

--Evan in Paris