The problem with going from Sony to Amazon is that all your ePub will need to be converted. You will have to strip the DRM (if you do not already) and then convert. You won't have access to library ePub (if you do not strip DRM). So if you are one of those who do not strip DRM, the K3 is not a good choice. The 650 is the best choice for you given what you'd have for eBooks.
If you do strip DRM, then the K3 could be fine. But, given that you can turn in your 505 for a credit, that drops the price of the 650 down a bit.