Could you show us the ones your talking about. And what about their prices.
Well for 1 im pretty sure xfx is known for having 4000+mem clock because pretty much all there cards have it, im talking bout xfx radeon 5830 shogun 2 bundle on tigerdierct, its a featured gpu and is regularly 130$ 4 limited time its 99 ( till may31), the saphire is the 5830 and is on newegg for 110$.
The two cards have the same memory speed. The listings just use a different way of calculating it. The Sapphire card is better for overclocking. It also has no mail-in rebate to bother with and wait months for. Unless you are really interested in the game it is what I would go for.