I've used both so I can compare.... from my experience, Nan is better and much closer to breastmilk.
I gave my son Nan at first after my family doctor told me it was the closest to breastmilk. After 3 weeks, I gave in to my mil who believes that Similac is better. The first time, my baby finished the bottle but obviously didn't like it. The 2nd time I tried to give him Similac, he flat rejected it. Now I'm back to giving Nan.
I actually tasted both brands myself. Nan is not fantastic but Similac just tastes horrid. There is nothing sweet about it.