Hi Sophie,
Two things to think about, firstly I'd bet everything I own that the products at CND are far superior to those you'd be using with Sally's. When you are training you need to pinpoint the problem areas. You need to know it is your method or application that is the problem, not the product. With CND you know that, if you encounter problems it's technique or prep, not the products letting your down. Secondly, I have heard a few people complain about Sally's courses on here, or regret going to them. I have never yet read anyone complain about or regret their CND training, and I think that says a lot.
Good luck making your choices but I think you would benefit from a lot of research on here before you settle on either xx