Trying to compare number of lines on different readers seems to be very difficult as it isn't easy to get the font displaying the same, especially when using readers from different companies.
My suggestion is to compare the physical space available for text on the different readers. I recently did just that, see my post Aura vs Paperwhite for measurements taken on the Aura. However I no longer have a 650 so I can't provide these measurements. Then, assuming you would use the same (perceived) size font to read with, this should give a good idea as to how much more text you would see per page on the Aura.