Want to get my wife a mountain bike for our wedding anniversary, looking at something like a Cannondale trail sl 3 or Trek 6300wsd. She is 5'9" with a 34" leg, not sure what size to go for.

Go to a bike shop. Ask the staff. Try the bikes out for size.

Is it really that hard?

I'd echo CF's comment , don't buy her a bike that she hasn't at least tried for size.

For what it's worth mrs carlos is 5'6" (32" inside leg)and rides a mans xs F700 'Dale

I'm sorry but your good lady rides an "mans xs" 🙂 hahaha! Anyway your 100% right, get her to a shop. She's not going to like it if it doesn't fit right and if she doesn't enjoy it whats the point?

Oh yeah and maybe get a hub gear? Loads less faff, means that you don't need to explain chainlines and shifting under pressure etc on ride's. In my experience that just add's trouble. Already she's going to be nervous about coming out on your thing and if you stand and lecture her for 20 mins on how to ride a bike she's just going to feel worse and not enjoy it. I don't mean to be sexiest btw, i have been dropped by pretty much every girl i have ridden with in a group ride, i know girls can ride bikes better than me! I just mean to save arguments with your good lady so you both enjoy it! 🙂

If she's long limbed, like my wife, she'll be better off with a 'man's' frame. The geometry on women's frames is a bit weird. My wife's 5'10" and rides a 19" frame, which fits her perfectly. Don't forget that top tube length is crucial.

My wifes, 5'8", 34" legs and recent went from a man 16" frame that I originally built up for her, to a Womens specific large frame (Giant Arete from Paulscycles) which fits much better.