I traded my 505 for a Nook wifi about 2 weeks ago. I thought about it a long time while asking for opinions on this site. I first decided not to switch since there is nothing wrong with it and I just loved it so much. However, once the Nook wifi came out for just $149. I just couldn't resist. I pretty much switch because of the more frequent firmware updates and the abilities to be hacked. To be honest, I am probably not going to ever hack it unless I am out of warranty. I am using the Nook only to read fiction books and not going to do much else but I'd to have all the features just in case I might need it someday. You just never know. The two devices are virtually the same other than the touch screen which I have come to love! Shopping via wifi is awesome even though it's something I never thought I'd like before when I owned the 505. Also, swiping page turn is a pretty nice touch. The screens have about the same contrast. There are some things you might have to get used to IMO:
1. The weight (with cover on) is for me significantly heavier.
2. It's also real bulky compare to the 505. I love the slimness of the 505 itself and the included brown cover.
3. No collections option as you might already knew - I only have about 100 or so books so it doesn't take that long to browse through them. Plus I can just go to reading now and get on reading since I always read one book at the time.
All in all, I love the Nook. If I have to do it again, I'd do it. The most awesome things about the Nook in my shallow opinion are the look (nice, futuristic with the touch screen, and sleek) and the abilities to accessorize!
I had a hard time finding the cute accessories for the 505 since I bought it 8 months ago. I now own 2 skins and 2 cases for the Nook. Loveeee it!
Anyway, good luck on your decision!