This is a real easy question. There is a screen reader called Gnopernicus for Gnome in what I'd call early beta testing, and KDE has no screen reader for it. Thus, the choice is kind of limited. I wouldn't mess with either unless you feel like doing beta testing of possibly very unstable software. Yes, it exists in Linux too. The difference is, unlike Windows, people actually fix the bugs rather than just patching more and more code on top of them.
