You're right, that Zire is terribly cute and attractive. However, both run Palm OS, which is notoriously user-friendly, so the end result is both should be equally easy to use. I'm not sure that 2 rather than 4 buttons increases ease of use (that's two things you now have to go to the home screen to launch rather than pressing a button).
The fact that the Sony happens to have more memory and an expansion slot really doesn't change the way your wife would intereact with it. You never notice memory on your computer unil you don't have enough .
Rechargeable batteries are more user friendly, IMHO. The screen is better on the Zire too- brighter and doesn't have the ghosting the SL10 shows.
When it comes down to it, you know your wife best, and if the attractive and simple package along with rechargeable batteries seems like just the thing for her, I say go with the Zire. Specs aren't as important for some folks. Just keep in mind that both are equally user-friendly.
-------------------- Lisa Gade Editor in Chief, MobileTechReview