As someone who has also had both, the Sony product is much more finished. My Kobo was missing all kinds of features that would be nice to have in an ebook reader. Such as turning to a specific page, or using the build in dictionary with any document, not just those purchased from Kobo. The kept promising features like these in "the next release", but honestly I hate it when I buy something unfinished and have to beg the company to do what they said they would up front.
Also, my battery was always going dead after a few days - much, much sooner than the Sony.
Lastly, my Sony reader is compatible with many more formats than the Kobo, and my Kobo library transfered over seamlessly.