Depends on price. I had a 2014 yz450f and liked it but had random starting issue that couldn't be figured out. It took a lot of tinkering to get the suspension dialed. The 15 is suppose to handle better and 16 is an all around better.
I never owned a 14 350 but I currently own a 15 and like it a lot. The power is perfect. The 14 350 was down on power and the suspension isn't as good as the 15.
Not matter which year of Ktm you buy get one where someone has figured out the suspension. The suspension is the kyrptonite of all Ktm.