Is sideloading a realistic option for you? If so, then stick with the reader that you most enjoy using and manually load the books that you purchase/download. Both the Nook and the Kindle can be jailbroken/rooted to allow all sorts of different non-native formats. Also, Calibre is a great tool for converting between formats.
As long as you have computer and internet access, this might be your best option. Otherwise, I think you'd want the Kindle because of the international 3G coverage. But then again, I have been considered something of a Kindle fanboy
