I would just like to add that on the 3ds xl, both screens appear to be larger than the iphone screen, and both screens together are nearly the size of an ipad mini. Not sure if that's helpful, but, I think the take away is that the 3ds is much easier to read from than its predecessors and comparable to apple products.
As for the game itself, you know your mom best. If she's more into collecting, go for New Leaf. If she's more in to the decorating, maybe go for Happy Home Designer. Or just let her try both, I guess?
Hmm, I feel like the iphone 6s plus has a larger screen than the DS. I do know that shes happy with that size though so I reckon it should be fine.
I think what I'm going to do is buy her her own copy of new leaf and let her have my Happy home xD
@Akibear - my mom isnt into video games either but shes plays a lot of iPad games though (all very relaxed and either to do with decorating or collecting). I play some of them with her but ACNL has more player interaction than the ipad games.
@m3ow_ - if you happen to remember, could you let me know?