The two e-readers are almost evenly matched in the physical features department. The only variable left out is the quality of the screens.
Both have great support structures (e-book stores) that may differ between several countries around the world (in my own country, the Kobo has two online stores that sell localized - translated - ebooks, while the amazon is rather lacking).
So I am going to compare only the devices technical specs.
Pros for each one (mostly taken from here):
28g lighter (13% lighter than Kindle)
has Micro SD slot
dedicated light switch button
USB mass storage (can connect to thumb drives?)
has a 3G version (I won't get a 3G e-reader though, but as reference)
WHITE E-PAPER DISPLAY
Now, the real question is:
I had not the opportunity to put both devices side-by-side in front of me to test their screens. What is the contrast ratio on those two devices?
Bonus points if answer points me to tech sheets with the contrast info.