I'm have the same question to some extent, and it's still a valid question even if it's going to be crappy on all of them.
Obviously the biggest screen you can get is better. If he's not commited to eink, then there are lots of bigger options. If he's like me:
I want a small eink screen that I will use for other things, but still question which ones handle PDF best, i.e.:
which ones support landscape mode (which will obviously be significantly better)?
which ones give you flexible zooming? (I've heard some only have fixed width, fixed height, and original size). I've read that some won't even let you pan around, so even if you're just trying to look at a one page document and be able to slowly find all the text on it, it still won't be possible.
Will any automatically zoom through so that the margins are outside of the viewing screen, making the text slightly bigger?