Get the 650 and you won't look back and you might even want to then get your mom one too. Getting a 650 means all the eBooks you have now do not have to be converted and can be used as is. Also, the interface on the 650 is very nice. The touch is well done. No touch layer and it uses a Pearl screen.
IMHO, Sony has the best screen given that it is Pearl with IR touch. You don't even have to calibrate it.