Both are really nice lenses, especially given the price point. The Nokton is full of character with more of a classical look, and the Ultron is clinical and sharp, more of a modern look. The Nokton, however, is sharp enough, and I was only disappointed once, when I used it with Tri-X and Diafine. Both are light weight, small. Build quality of both are better then the Leica Summicrons I once owned (4th and Asph). I prefer the ergonomics of the the Nokton, but that doesn't mean the Ultron is bad in this regard. It just takes some getting used to. Since I'm a bokeh lover the extra stop of speed on the Nokton is fun to use. I see why the pre-asherical Summilux has such a strong following. The crazy, erratic bokeh wide open is a good metaphor for life. 🙂 The bokeh of the Ultron is a more modern, smooth transition. I've used the Nokton much more, because the camera dedicated to the Ultron has been sick.
Sorry, I'm no expert with optics, and just now getting back into M mount cameras after a long absence. I realize my observations are a bit banal and trite, but we're fortunate to have such choices at relatively affordable prices.