Thank you all for your advice, I ended up buying a used, ad-free PW5 on Amazon for a decent discount. So far I'm pretty happy with it, though I'll keep my PW3 around for a while before selling it, just to be sure I won't change my mind
In case anyone is interested in a comparison, I'll say that the display looks fine - if there is a difference in sharpness with the PW3, it's pretty slight, and the lightning is even. The flushed screen does seem to attract dirt and dust more easily than the recessed one but I don't have much of an issue with fingerprints so far, even though I do a great amount of dictionary lookups (as I mentioned I use the Kindle exclusively for language learning). What I do like best though is how much quicker it is, again for the purpose of looking up words, switching between multiple dictionaries and highlighting. It was worth upgrading just for that, at least for me.