Exactly the same question i asked her (i.e. what does she actually like in the M5)... but i could not get an answer...
Perhaps its due to the following reasons
1. Good front and rear camera
2. 3gb ram (it seems like she really wants a minimum of 3gb ram, but 2gh is "ok", whatever that means haha)
3. Design
4. It's a sony (she really loves sony, i dont mind her being a fan of sony at all.. it's her choice

)
5. A screen that is not too big (she is a bit petite and small, she does not even like my 2 year old samsung note 3 because the screen is too big)
6. Probably the price (i will be buying with my money and not hers... its just that she is a loving wife who does not want me to unnecessarily spend money)
Her main usages are
Whatsapp, instagram, Facebook, calls and sms, youtube and videos (using mx player), simple never ending run games and baby games..
Does this help?
It does, sure.
But regarding the Sony fan part, it wouldn't allow much choice since other that the mentioned XA phone (as size need to be small), only Xperia X is available around that price with good selfie camera.