As long as they aren't heavily loaded with pictures like some history or cookbooks are, you should be able to squeeze up to 6000 books onto a 8 GB memory.
But a full memory makes the device slower, so I am not sure I'd do so.
Anyway, we live in a world with free wifi in nearly every coffeeshop and cloud storage. Both do connect with Dropbox so there is no real need to carry your whole library on the reader. I have about 1000 books on a reader most of the time, equaling about a decade of reading.
The Forma is available with 32 GB (I saw it for 300€ on ebay or you could buy it for about 335€ in the
official British Kobo store).
I didn't like the Forma and sent it back. I read lying on the couch or the bed, nearly never do I sit down to read and the ergonomics of the Forma didn't work with how I need to hold the reader. Also I think the Forma should be used without a cover to benefit from its form factor and its extremely light weight, but when you do so, you have to switch it on AND swipe the screen to get to read with it. This drove me nuts. With a smart cover of course it goes right into the book or onto the home screen, if you didn't shut it mid reading.
If I were you and there is no store where you can look at both devices, I'd make sure to order at a place with a good return policy.