As has been said before, you CANNOT compare the 505 with a Kindle 3 -- they are different generation devices. The correct comparison is the Sony 350/650 to the K3.
The new Sonys have the same Pearl screen as the K3. There is no difference.
Although the 505 had very limited zooming, the new models have much greater zooming capabilities. The touchscreen, which was problematic in terms of clarity on the predecessor 300/600/900 models for some users, is now as clear as the K3 screen. As for weight, the new Sonys are much lighter (or at least feel that way) than the older 505. The K3 and the new Sonys are probably similar in weight but are not similar in ergonomics.
The questions you need to ask yourself are these:
- Do I want to convert my current ebooks from ePub to Mobi?
- Am I able to strip the DRM from any ePub that has it? (Yes, I know there are tools available and many MR members do it regularly, but the question is whether you are capable or comfortable in doing it, not that others can do it.)
- Where do I prefer to buy my ebooks? (If you prefer to buy your ebooks at Amazon, then your problem is really solved, buy the K3; OTOH, if your favorite ebookstore sells ePub formatted ebooks, you choice should be the Sony, Kobo, or Nook unless you prefer to strip and convert everything you buy.)
Good luck. You really won't go wrong with either a new-generation Sony or the K3 in terms of reading. I read for 4-5 hours every day on my 950, just as I did on my 505.