Best bet is to go to the actual store and give her the chance to try out various handsets, and decide for herself which one she likes based on in-person hands-on experience.
My wife loves her S2. We did originally buy her a Pantech phone with a slide-out keyboard, and she loved it for a while, but the screen was too small and the phone was too slow. Thankfully the discounted plan I get at work through AT&T throws upgrades at her every few months, so she stopped by a store and went hands-on with a dozen handsets or so, then chose the S2 for her birthday this year and loves it.
She's got another upgrade available and has tried out the S3, but she's very happy with the S2. The S3 would probably be too big for one-handed operation for her.
In my case, my big meaty paws can barely tolerate the Thunderbolt/S2 screen size and I can operate my Nexus 7 one-handed, so the S3 is what I'd love to have. I may have her use her upgrade for an S3 so I can get a month-to-month AT&T contract on it (or maybe even look into StraightTalk SIM or something), and she can continue using her S2. That way, we're locked into only one contract. LOL.