Sorry for the late reply,
As a ZBrush user, I can say that pixelation would be something you can work with. After the latest update is not too bad. I have a large iPad Pro. And is still light enough that you can use it. Heck, much lighter than a cintique companion for sure.
The thing is that as ZBush workflow goes, it would be hard to work just with the iPad alone. Unless they make some serious update on the controls, you will need close by a keyboard. I had to discard the sculpting from my couch idea a while back. This is not a lack of features with Astropad (although more features are welcome), but the fact that working on ANY 3D package requires many keys to play with as the controls and options are overwhelming sometimes. So you are bound to have a keyboard with your iPad.
So if you already picked the larger format of the iPad, good! If not, a bigger real state is always better.
Pixelation is something we have to deal with in ZBrush, but is not as bad as it could be.
Astro pad would not be a good fit yet with 3D programs as Maya or 3DS Max as they need the constant refresh and the whole program will pixelate badly. But again, I didn't bought this product to do that kind of work with it.
hope this was of any help to you or anyone thinking in the same lines.