Welcome to MobileRead, gratin3!
I think you'll find that most of us love our reading devices around here -- but we've all got different ones!
Seriously, though, the reason most of us love our readers is because most of us have done what you're doing: researched the matter before-hand.
The Gen3 and PRS505 are fairly comparable machines as far as hardware goes. The displays are the same size and the same generation of e-ink. The processors and memory are similar.
What it really comes down to is what you want to read, with a bit of a question of how much conversion you're willing to do.
I'd suggest you start by looking at the various e-tailers for e-books and see how what you find compares to what you want to read. Obviously Mobipocket is the format you'll want to check for the Gen3 and Sony's BBeB for the PRS505.
You can find Mobipocket books in a lot of places, but for BBeB, you'll need to check
Sony's store and
Fictionwise (they sell some of their titles in LRF, which is the unencrypted variant of BBeB).
If you're willing to do some conversion, .LIT (Microsoft Reader format) can be converted to either format pretty easily, and it's very widely available.
I'd suggest you also have a look at the discussions in the "Which one should I buy" subforum here, there's a lot of good comparitive discussion there.
That should get you started, anyway, but do ask specific questions that you don't find the answers to.
The important bit is that you find a device that works for what you need, and the research you're doing is a very good start on that.
