Neither. Soft bindings are for people who like the park. They flex more and give you more forgiveness on landings. Stiff bindings let you transfer power quicker so you can dig into your carves. I'd say you should buy based on where you plan to ride. A mid flex could be a good compromise.
Union Force SL's for you, Union Trilogy for her. They're responsive bindings that are great for handling the entire mountain. If you dont want to pony up the cash for the SL's check out the Data or the Force. Union make some of the most comfortable bindings on the market. I'd stay away from softer bindings, I personally find they feel cheap no matter where you ride them.
390s are terrific bindings, but they are mid flex. They are mainly for people who want to ride park and mountain a bit. If you want all mountain, go for the Arsenal bindings. They are super responsive.
I know this post is super-old but stumbled upon it and wanted to clear this up so that beginners didn't stumble upon this and get some bad advice.
Beginners should never start with stiff flex!
A stiff flex binding is great for responsiveness for those riding fast down steep slopes in deep powder in the backcountry.
But it is difficult to control and there is very little margin for error. This is fine and works perfectly for an advanced rider that has that riding style.
But definitely not for the beginner. Your progression will be very slow with stiff bindings as you will have a very hard time controlling the board and you will be punished for every little mistake. This isn't a good way to build up confidence!
A medium-to-soft flex (3 or 4 out of 10) is a much better alternative. It will be forgiving and make the board easier to control so that you can learn and progress faster and build up confidence but won't compormise too much on responsiveness (like say a super soft 1 - 2 out of 10 flex).