I have used Kobo Touch to read PDF books and it works good for having only 6" eink screen. Kobo Touch much faster processor than Kobo wifi makes this usable but not great. Sony TRS-T1 reflows text and has pinch zoom making it a little better for reading some PDF books. Some PDF books and documents or comics really require larger color screens to get full experience. Advantage of black and white e-ink screen is battery can last for days to weeks of reading in full sun light.
Which one should I buy?
https://www.mobileread.com/forums/forumdisplay.php?f=123
This site has good reviews on tablets and ereaders.
http://blog.the-ebook-reader.com/