I've messed around with the GP5 and have owned a GP 4.2 for a few months. GP 4.2 is great for PDA functions, music, and pretty decent for things like reading, .pdf annotation, Evernote, and a few others. The 4.2 inch screen is perfect for fitting in any pocket. The 5.0 gets a little big for some pockets, at least if you're planning on putting anything else in alongside. The battery life on the 5.0 is reputed to be excellent. Having a GPS is also nice.
If you're thinking of reading books in the dark, keep in mind that the menu and back arrow keys light up when you turn pages, and that is a bit blinding in a dark room. I don't believe this can be changed, but you may want to look into it more. This occurs in the Kindle app, but perhaps it could be avoided in an alternative reader.
Compared with the Nexus 7 and the KF HD, the GP's are a bit overpriced, but the small size and SD card expansion is really handy.
The above post re: the unlocked Galaxy II phone is also good food for thought.