My opinion is that you really can't go wrong with either. Like most modern eInk devices they generally all "just work" when it comes to reading.
Another difference I didn't see you mention is the type of touch screen. The Glo HD is IR while the PW3 is Capacitive. If that makes any difference for you. Some folks who do a lot of highlighting & stuff fine IR less accurate.
If I personally was choosing for myself I'd get the Kobo. I've had both Kindle's & Kobo's and find I like the extra adjustments I can easily make on the Kobo (line spacing, margins, fonts, etc.). I do find that I dislike the Kobo's home screen and wish it could be eliminated in favor of the library screen (maybe there's a hack for that).
Good luck