Those tablets will be fine. Basically, when you look at Zbrush, you want a pen, any pen. try to get something with at least 1024 sensitivity, 3 buttons is nice as well (tip, and 1 rocker). I believe all bamboo pens come with this now. After that, size is really up to you. If you make big broad strokes when you draw then you’ll want a bigger tablet most likely. If you make smaller ones than a small tablet is usually fine. If you use 2 or more monitors you may want a medium to large sized tablet because you’re forcing your tablet to work over 2 screens. If you’re okay with using a mouse and a tablet together then this isn’t really a big deal. I never use a mouse any more unless I’m playing a FPS.
I have a bamboo pen and touch (whatever the $99 one is) that I keep in my bag with my laptop. I have an intuos 4 large that I use at work and both of them area great.
Hope this helps