The only reason to upgrade would be if you've had a super extensive ebook library and want to take it with you all the time. I checked my Calibre library, and it's 7.2 GB in total, which includes some manga from HumbleBundle.
Sideloading audiobooks on the Kobo is a pretty shit experience. So if you only take Kobo native audiobooks into consideration, you probably still don't need more storage. Unless you're preparing for some sort of end of the civilization scenario, in which case an audiobook library would probably be your least concern.
