They mainly vary in screen quality. More expensive readers have higher-resolution screens with higher contrast.
The new Kindle Paperwhite is probably the best all-rounder and the newest version is fully waterproof.
PDFs are a bit awkward, because they're generally formatted for A4 and don't display well on a smaller screen. There are large-format e-readers available from Sony, but they're cripplingly expensive. If you read a lot of PDFs, you might want to consider a 10" Android tablet instead.
Good advice, thanks. I think many of the PDFs I read use a different page size, so I may get away with the e-reader. Funky formatting is probably inevitable, even with a tablet.
>>26991 Yeah agreed - I have the "normal" Kindle Paperwhite, not the posh version shown in the OP - it is simply brilliant. I love the fact it only does one thing, really well.
Thirded. I can see much of a reason to get any of the pricier models. The Paperwhite is excellent and the form factor is perfect. Really don't understand why you'd want a chunkier, more expensive model.