I have taken First a couple of times, but generally take 2nd. It's still perfectly comfortable, and usually quite quiet. Around holiday times, it makes sense to take first if the price difference isn't too big since the 2nd cars can be loud and fairly dirty. But other than that, I'd probably stick with 2nd. More times than not, I had nobody sitting next to me and/or I was able to move to have nobody sitting next to me in 2nd.

Also, avoid IDTGV. I once accidentally got on one instead of my train (it was one of those times when the trains were linked together) and it was absolutely disgusting. Maybe it was just bad luck though, since it was holiday time and I was going Aix-Marseille (the last 15 or so min of the Paris-Marseille route).