Beryl is a Window manager, gnome and kde are a collection of many different applications a WM is just one of many they provide. 🙂
It just depends on what you like more. Personal preference. That's all.
Different people like different things. Personally I like Gnome, I think that it's simple and once you get past gconf and such things it can be very flexible. I don't like KDE, it's ackward and I don't like the configuration utility of a thousands windows of conflicting and confusing functionality.
But other people like that stuff. They think KDE is wonderful and is pretty. They like Konqueror browser-style file management.
If you want me to choose I'll say try out Gnome. KDE is working on KDE4 which is a dramatic change from what people currently are using. So muck around with Gnome to get familiar with that until KDE4 starts being usable then try that out and see if you like it.