What is the difference between 1st and 2nd class seating? I'm not sure which one to make a reservation on.

In first class you get a slightly larger seat and 2 x 2 seating with a center dividing aisle. You also get a little more leg room.

The seat itself is a little larger.

I don't think the difference is that dramatic. For me it is not the big difference you get between economy class and the next higher class on a trans Atlantic flight.

The question on the train is this:
Is the slight upgrade in comfort worth the extra cost??

It's all a matter of comfort. Personally, I prefer to book the 1st class tickets on trains and ferries. To me, they always seem worth the additional cost.

In first class TGV between Lausanne and Paris there are also single seats which I love, nobody hogging the arm rest. And so much leg room. When I go onto the SNCF website to buy tickets I frequently get a highlighted note saying that for 10 euros more I can buy first class and when it is that small a price difference I always do. Worth it I think.

I usually book 1st class on TGV trains if the price differential is no more than 20% higher. Note that there's no free food in 1st. Someone else might be able to confirm, but it's also possible that 1st class compartments have AC in the summer while 2nd class don't (I've never taken 2nd class in the summer, so I'm not sure.)

No -- all TGV's are air conditioned. It makes headline news whenever the AC on a TGV breaks down during a heatwave, because anybody who has seen the doors close at departure on a TGV knows that those things are hermetically sealed like an airplane.