I don't think she would either. I have one and
1. I'm trying to justify whether it's worth the extra money for a 6P for her.
2. I really don't want her phone to look just like mine so she doesn't start futzing with mine.
She has dropped the 6 countless times so durability is an issue, as is size but they they are both smaller than the 6. We use Project Fi so it had to be one of the two.
The 5x is extremely durable. Mine has been around the world with me, from hiking places like Alta peak in California to travelling around Asia. All with just a screen protector. I love the materials they use because I hate cases so it works great for me if small scuffs on the plastic don't ruin the look of the phone.
If she really doesn't want to lose the size and you are worried about you two having the same phone, make sure you order her a different colour 6p haha.
If you are most focused on price then I'd suggest waiting another month and a half for the new Google phones to come out then get the 6p at a discount (or even better get yourself one of the new ones and give your wife the 6p)!