Asus always, With Asus do expect a better life expectancy and better heat dissipation. For less, Why spend the extra for a not as good supplier? Different brands sometimes do matter as some don't have very good coolers on their cards but for the most part, their fine.
There wont be a massive difference in quality, going through different Brands, although some 3rd party coolers might be a bit higher quality/performance than others. Brand should only matter to you, perhaps you're loyal to one, or you like the warranty from the other, or perhaps you like the cooling solution one has. In this case though I would give Asus my business just because they are extremely reputable, more so than Diamond.