I'd have to say that ultimately the most important feature is having books to read. Since DeDRM is not an option for you, that gives the Kindle an advantage because of the superior store -- so I'd probably go with that.
As I said above, IMHO I don't see any problem using plugboards for series information (and of course you can add books to calibre and modify the metadata even without removing the DRM).
And the Kobo only "seems" to have a longer life, I wouldn't put too much faith in that. It's kind of tricky to really prove one way or another, and I doubt you have done comprehensive testing (has anyone, actually?). Besides, either one will last pretty long anyway.
...
Yes, I
do have a bias toward the Kindle. Doesn't make any of it untrue.
