I doubt you could get something with a significantly better screen than the Bebook - at the moment. Some have reported that the 5" devices appear a little sharper/crisper because of the relatively higher resolution (200 dpi vs. 166 on the 6")
If you are going backpacking, I would value portablity highly. For that reason alone, I wouldn't recommend even the new iRex if you could get in time (I don't know where you live??) - but it's also a bit unknown yet how well it can perform.
You are probably already aware of the limitations with regarding reading academic PDF journals on a 6" screen

In my experience you can't go below 10" to get a good reading experience - so perhaps you should prioritise portability and price and get a small cheap reader and then splurge on a large one in a year or two? I agree with you that we're still in the middle 90's with regards to ereader technology.
Have you tried converting PDFs? It depends enourmously on the source material how well it converts. One column, text-only, no footer and header works (very) well. The more fragmented the page are, the more likely you are to get a bad result.
A 5" screen
is somewhat smaller than a 6", but if you are comfortable with reading smaller print, then perhaps it would still be okay. Personally I would not like it because I prefer at least 12 pt text, but I know many reads 10 pt. I don't have personal experience with 5" ereaders though.
You could also go for a Sony 505 - they're about as cheap - or cheaper - than the new 5" devices, such as Sony 300, Hanlin V5, Bookeen Opus.