The 5" Hanlin is better looking than the 6" and has updated hardware. The standard firmware is virtually identical to the 6". See Review of Hanlin V5 for V3 owners
for a more details (note that Hanlins now have more font sizes, up to 9, so my comments about the available sizes is out of date).
I use the 6" because it can run OpenInkPot, which has not been ported to the 5" Hanlin. There is a trade off between screen size and number of page turns, but this is only a large factor if you expect to use a large font size. I use a relatively large size, and found myself reading in landscape mode on the V5.
If you are buying new, the 5" Hanlin is the best value for money. Possibly a bit more expensive than the PRS-300, but with more capabilities. I assume if you are buying used there will be more 6" Hanlin V3's available than the relatively new V5.