I had a similar issue to consider when I recently moved from a Sony PRS 350 to a Kindle PW2 -- convert epub to mobi or epub to AZW3. I chose epub to mobi for these reasons:
1. I mostly read fiction and so mobi's poorer ability with style is almost irrelevant for me.
2. I sometimes read from my phone. My phone is always with me whereas my Kindle isn't and being able to just open my phone and start reading exactly and almost instantly where I'd stopped on my Kindle is a huge bonus. This, as far as I know, can only be accomplished emailing mobi format to your Amazon account or with a store bought, DRM intact Amazon book.
Hope this helps.
An after thought: I a world dominated by propitiatory technology silos, mobi is a more open-source format than AZW3
