As someone who has used 34" 3440x1440 both flat and curved I tell you, you definitively want a curved one.
I had a flat one at work, and when my colleague went on holiday, I stole his curved screen... never gave it back
. I was worried as I do a lot of screen design stuff, that on the curved I would not see the straight lines straight, but you get used so quickly and from the first minute on, it was clear that I would stick to the curved, it is so much better.
Before getting the 34", I used 2 x 19" @ (can't remember the resolution, but it was more than Full HD). I most definitively prefer a single large screen, as you can have your main working area in the center and other stuff to the sides.
Size wise, I find the 34" comfortable and don't have to move much to see all, larger I find not much of an improvement anymore (My boss has a 49" 5120x1440 , which I stole too, when he was away, but gave it back). If I were to upgrade then to a 34" 5120x2160.