I have riden both Union and Rome bindings in the past, but I am not familiar with the Nitro bindings. The Unions are great in their simplicity, but I find with long and hard days the comfort level is minimal. The Rome bindings are great for longevity as well and if you can find a pair with their "yes I cant" system the stoke factor would be high. They are a touch heavier, but not enough that I noticed. I have been riding the cant system and found that I can charge much longer and harder then without. The Nirtos are a great price and I feel like they are one of most underappreciated brands the consistently turn out quality products.
Either way your lady friend should be happy to have a traditional system.