Prefer the Z-series
Came with better screens and nVidia graphics. Light machines which run CUDA stuff. Not cheap though. My even older Vaio SZ is still working, and even it can run more (older) CUDA stuff. No Ultrabook ticks that box.
Fortunately, there has been a spate of very decent 13" and 14" laptops with nVidia 520 and 540 graphics on board. Cheaper than the Ultrabooks too. So, guess what I will get to replace my crumbling SZ.