Diamond has done a wonderful job with their X1900XTX card, in that the card is an ATI reference card through and through. Virtually every X1900XTX manufacturer has followed ATI's reference design for the X1900XTX cards, in PCB layout, cooling, even down to having Ruby on the heat sink. The clock speed on the Diamond card is exactly the same as the GIGABYTE ASUS and ATI cards.
Diamond differentiates itself from the competition by having a Help Desk application on the Driver CD, a 1 year warranty on their video cards and relatively up-to date drivers on the Driver CD and the card is available with a $100 instant rebate from the Diamond Online store, bringing the final price of the card to $499, the same as NVIDIA's 7900GTX and in-line with the retail prices of other ATI X1900XTX on places like Newegg.
If you're in the market for a high-end video card today, the X1900XTX of any flavor is a great choice, as the card is the fastest, most feature-rich available. It also has the best image quality when Anisotropic Filtering is set to Quality mode. I prefer NVIDIA's Transparency Anti-Aliasing to ATI's Adaptive Anti-Aliasing, but that's a matter of taste. Add to that the fact that there are few, if any, 7900GTXs on the market at the moment (the card is selling almost as soon as it arrives in e-tail), and the choice between the two is pretty clear.
