hello modern bear and welcome to the forum ! we are glad you found us too !
what a nice husband you are to buy your wife a reader for her birthday ! she is lucky to have you.
the sony readers are excellent machines and you'll find plenty of people around here who absolutely adore theirs !
the problem with pdfs on readers is that they are often formatted for a much larger display than a 6" screen, and although the sony pdf support is very good and will allow you to "reflow" the pdf text (this means it will adapt the text to the size of the display), when you do that you lose most of the formatting. but, while you're waiting for an affordable large-screen device, the sony can be a very good alternative.
i personally have a sony 700 ; it does have a somewhat inferior display to the 505 because the tactile layer plus the gap for the integrated lighting somewhat obscures the e-ink screen (which is below those). however, it suffers most in a side-by-side comparison ; when you actually use it to read, it's an excellent device and the screen is absolutely readable, i've never had any trouble reading on it even in low light conditions, and in daylight it's even better. generally, people who are already used to reading on a 505 etc. don't like the 700, but those who get it as their first reader generally love it. i have to say that the ergonomics are absolutely brilliant, and the touch screen and note-taking ability (and search and highlight...) make a huge difference and for me they outweigh the (slight ! really !) loss of contrast on the display. which is not to say it's perfect, of course ; when someone makes a device with touch-screen navigation, note-taking, etc. AND a display as clear as a 505 screen, i'll definitely upgrade ! but in the meantime i love it.
the other very important feature to consider is format support. the sony devices support epub, which is the new industry standard format and already gaining a lot of ground, although mobipocket (which has been around longer) is very popular still, particularly in the US. personally i wouldn't get any device which doesn't offer native epub support.
whatever you decide i'm sure your wife will love it ! take a look around the forum, and don't hesitate to ask any questions.