Different focal length, you want something wider then go for the 30mm, especially after accounting for the crop factor on DX sensors if you want to approximate the view of a 45-55mm standard focal length. 50mm might seem longer in that respect.
That depends very much on the scope of usage. Given that the 30 f/1.4 is a DC lens, I would assume usage is on a 1.5x camera, which means I'd pick the 30 f/1.4 for general purpose, or the 50 f/1.4 for portraiture.