Have you considered or heard about the new Nook
that was just recently announced and will be available on June 10th? You also didn't state which Kobo you were considering since they just released a Touch version.
Kindle is a eco system like Apple devices which has it's good and bad points. But some of the newer eReaders just released give the Kindle a run for it's money. Unless you mainly plan to use Amazon for your book purchases.
To convert files to various eReader formats you can use Calibre
- it's free. But with purchased books you'll need to remove the DRM before you convert to Calibre, which is a whole other ordeal.
Kobo eReaders typically support reading-centric only features. But the new Touch version ups the bar. From what I've read - neither the Kobo or Kobo Touch have a web browser.
The Kindle 3 has a web browser but you might have problems with some websites. But you can use a website like Kinstant
which will give you a jump page of sorts to mobile versions of websites you want to visit like facebook. Just don't expect the same browsing experience as with a PC since you'll have to use mobile versions of some websites. Facebook -> touch.facebook.com or m.facebook.com will give you an idea as to what your Facebook browsing experience will look like.