I own both, but I sideload from Calibre exclusively, never download anything directly from any store. I want my library all in one place, with cleaned-up metadata, the layout and formatting of the books edited to my liking. I didn't get several different ereaders because I was interested in easier downloading, but from pure gadget lust.
If you don't want to bother with library keeping and having DRM-free back-ups of your purchases is not a priority either, then by all means get a Kindle in addition to your Kobo. If you're just feeling gadget lust, get a Kindle anyway. Both Kindles and Kobos are excellent readers. It never hurts having more of a good thing.
