For me, the Kobo lacks something like the Send to Kindle feature.
I can send books I've obtained from outside the Kindle Store (DRM free sellers, Humble Bundle, public domain etc.) and then store them in my Kindle library.
These are then synchronised across all of my Kindle devices and apps - completely for free
Kobo does have integration with cloud storage, but it's not the same feature.
Plus it has the Kindle Store, which has the biggest selection of books for sale.