XFX generally has better warranty terms (double lifetime warranty, if you ever decide to sell the card), but they and Asus are somewhat known for being slow if you need to RMA. However, Sapphire and Asus are known for making very reliable products, and Sapphire is one of the best AMD GPU manufacturers (since they only make AMD GPUs, like EVGA and NVidia).
All in all, unless there are some glaring and widespread issues with a specific model/series of the GPUs you're looking at, any of them will be fine.