To be totally honest, I don't think you'd be happy with either reader.
The Kobo is buggy. Not the hardware, but the firmware is. The 505 is not buggy, but it uses a rather old version of ADE that does not support full justification and if that's an issue you won't be happy.
I've had a 505 and it was good for it's time, but it's time is up. The Kobo's firmware is not good and you won't be happy with the bugs.