I was in the same position you are now about 2 weeks ago, i decided to take the plunge and today i got my K3 3g/WF. I too bought it to try out as i knew i could return it in 30 days with Amazon paying the return fee.
As soon as i took it out of the box, i knew i wasn't returning it....
One of the first things i noticed was how aesthetically pleasing it was, it has a distinct minimalistic style made from lightweight polymer whereas the PRS 505 looks antiquated with it's metal shell. Lifting it out of the box i was amazed by how light it was, about half the weight of the PRS 505 i would guess. The profile is thin too about the size of a pencil.
They pack a usb charging cable with a mains converter adapter so you can charge your kindle via your computer USB or via the mains. Theres a little instruction booklet but it's not that useful.
Upon charging it for the first time a msg came up saying i could read books whilst it charged in the background, something the PRS can't do, i thought it was worth remembering that. The first charge too about 1-2 hours, not really sure.
My main gripe would be that as a new kindle operator there seems to be a lack of intuitiveness about it. I was confused about how to go about adding books, when confronted by this keyboard and menu buttons it took awhile for me to get use to it all. The PRS 505 definately wins here as everything you needed was in folder format when connected to a computer and it seemed easier to get started. There are no extra buttons but there are no extra features either on the 505.
I thought Amazon might have had their own software that i had to install but that doesn't seem so. I ended up using Calibre which i knew how to use from my PRS 505 days. Transfer of books were no problem whatsoever although i am still trying to port over all my books and making sure they all work.
One of the big questions i had was how restricted to Amazons formats i was going to be since i read alot of epubs. Calibre converted all my epubs in a few seconds so it ended up not being worry over nothing.
Experimental
That's right theres an actual category in the menu that says "experimental", within it lies the the web browser, the music player and Txt to speech.
The web browser is something like you would find on an old 90's computer but in black and white, the 3g and wifi is kinda slow loading pages, you would probably be better off using a cell phone or itouch if you want to browse the internet properly. Upon reflection, because of the slowness and basic browser, i don't think it's worth getting the 3g version of the k3 unless you travel alot and wish to buy books or get updates from areas not covered by wifi. so my recommendation is wifi unless you have loads of cash to blow.
My other gripe would be that the k3 music player doesn't seem to display a track list, so you have to always start playing music from the first track and press alt f to skip tracks or alt space to stop playing. The 505 has an edge over the K3 in this regard as it devotes the page to display all your music tracks.
Although the K3 has built in speakers and phone jack whereas the PRs 505 only has the phone jack. All in all i am disappointed by the experimental features, they seem to be very basic, half baked features. Hopefully Amazon will address these issues in a software update although i don't know how much support they will give it once the K4 comes.
Readability
The most important thing about an e-reader is how well you can read off the screen. Both the PRS 505 and K3 have very similiar screens. The main difference being the K3 have letters with darker contrast, i saw pictures comparing the two
and i would say it was very accurate representation. My problem with the PRS 505 was that sometimes it would seem like the letters were washed out especially in different light levels and it seemed that it varied from page to page.
I know there were posts about how you could open it up and mess about with the contrast settings etc but that was beyond my expertise. When i saw the picture above comparing the two e-readers i thought the k3 would look unnatural because of the darker contrast, but i when i held it up close i was actually pleased as it was now easier to read. I did not like the font on the kindle though but im sure someone out there has a fix for it.
Refresh rates, you notice a quicker page turn and page refresh on k3 but there is still a little lag when you quickly press the button so it's not like an instantaneous refresh but it's better than that of 505.
There are alot more hidden options about the k3 that i have yet to explore although first impressions are good. I'd say go for it just because of the lightweight reading experience and if your not too concerned about playing music or browsing the internet. I always felt there was something missing when i read on the prs 505, the K3 whilst not perfect seems to make e-reading a bit better.