I own a Keyboard and recently bought a Touch for my girlfriend. I've used them both, and I think they're both very comfortable devices. I've been reading with my Keyboard one-handed and find it comfortable, but I also have moderately sized hands. Still, I think a lady could use the device one handed.
Both devices (and the Wi-Fi only new generation kindle) are very light and easy to maneuver, and even with some motor deficits your mother should be able to use the devices just fine. Best Buy currently has all of them on display, at least in my area, so it wouldn't hurt for you to judge their size and layout yourself.
The touch requires a little bit more of a reach to turn pages, but not enough to matter, and it's certainly not an issue if you think your mom will be using two hands to operate the device.
I'd be more concerned with what will be more intuitive to her as a user. Not sure how technologically inclined your mother is, but I made the decision to get the 3G Touch for my girlfriend simply because she was hesitant to use the Kindle and I wanted to get her the one I thought would be most intuitive and immediately accessible. If your mom isn't very tech savvy, a touch screen might be the best way to go. Beyond that, I don't think you can go wrong.
Edit: Also, again, not knowing the extent of your mother's disability, and currently reading East of Eden on my Kindle, it occurs to me that the smaller buttons on the very edge of the Keyboard and Wi-Fi only Kindles might be harder to aim at and hit than simply tapping a large area to turn a page. Just another thought. Happy hunting.