My experience is that Superia 800 is nicer than Ultramax*. I find that the Fuji handles mixed lighting better, I think it shows less grain than Ultramax which I've found to be quite coarse. So almost the complete opposite of wblynch, above! Which, of course, leads to the inevitable recommendation: you're going to have to try a roll of each yourself.
Both films look nicer when rated at 640, I find, especially in the shadows, but you won't be able to do that with the Stylus.
* optical printing or home scanning.