Warning - this post is not 100% SS/FG specific

Hallelujah - my wife has finally shown an interest in joining me in the bicycle world. I am very happy to buy her a bike, but am cognizant of the fact that she may not stick with it, and think I should take that into consideration.

I've got it narrowed down to 2 bikes:

1) A fairly cheap IG 3-speed Mixte that should be more than adequate for her to get started on, but is practically worthless if she does not get into bicycling.

2) A pretty nice retro-modern Donna-style step-thru single-speed that is several hundred dollars more than #1. If it turns out my wife does not get into riding - it is a bike that I kind of like and it could come in handy for me as I am aging and developing joint problems that are starting to make it harder for me to swing my leg over my bikes lately.

BTW - it wouldn't hurt to have the extra few hundy to spend on my own bikes which have been trouble-free the past couple years but now are starting to need some work.

TIA for any advice/suggestions in this matter...