A couple of additions to the info already presented in this thread.
- My experience with my (now ancient) prs505 was similar to what was said about Sony readers in general. It is solidly built piece of hardware, firmware that come with it was stable and there were no major headaches with it. Their PC-based application was awful (a couple of new versions introduced, I still use it only for purchases or freebie downloads), but who cares, with Calibre around. One thing that is not pointed out is that Sony rarely does firmware upgrades. You buy a new model if you want the new version of ADE on the reader. Kobo reader firmware might be inferior to Sony at this moment of time, but if they continue to actively develop and distribute updates, that reader can easily end up with better software feature set in the future. It looks like Amazon provides upgrades for its older models.
- It is possible to format-shift from DRM-free ePub to mobi, but you might experience difficulties. It is my understanding that ePub feature set is a superset of mobi format capabilities. The difference is not important (Kindle books are nice "enough") until you stumble upon an ePub publication where the designer has taken advantage of, say, svg graphics. Can calibre handle that gracefully? No idea. You might want to do a bit of investigation (Kindle or Calibre subforums) if you decide to go that way.
- Ottawa public library uses Overdrive, and you should, if you haven't, check the collection to see how is it matching your taste and expectations wrt availability. It might or might not be a factor for your decision.