My vote goes fo eVGA for the links you provided. Overall, I have no preference over the two, but for the particular two you presented to us, the 7900GS eVGA is the way to go.
That card you linked to for eVGA is only higher in clock speed because it is overclocked past nVidia's reference 7900GS design. It is indicated by the "KO" at the end of the name, making it truly called the "eVGA GeForce 7900GS KO".
That XFX card you linked to is the non-overclocked version of the 7900GS which is the reason for the low clock speeds that are no match for the 7900GS KO. The highest overclocked version of XFX's 7900GS is this one: XFX GeForce 7900GS XXX-Edition
http://www.newegg.com/Product/Product.asp?Item=N82E16814150209
which is the one that should have been compared to the eVGA 7900GS KO, since these two are the highest 7900GS cards from each vendor.
Even though the XFX one looks great and has a lower price, the eVGA one is better because it is faster and still retains a low cost.
But XFX offers the fastest overclocked 7900GS, with the "Extreme Edition" and the "XXX-Edition" both beating eVGA's "KO" edition in clock speeds.
For example, my current video card is from XFX because XFX beats eVGA when looking at the 7600 series. The overclocked XFX 7600 (XXX-Edition) is 590/1600 in core/memory. The overclocked eVGA 7600 (KO-Edition) is 580/1500 in core/memory. And also, the XFX board looks cool, it's black.
However, XFX tends to have tons of different versions of every card they release. For the 7900GS, theres the normal version, "Extreme Edition", "XXX-Edition", and more, whereas eVGA only has one overclocked version, the "KO".
For example, the XFX 7900GS Extreme Edition:
http://www.newegg.com/Product/Product.asp?Item=N82E16814150202
is overclocked more than the eVGA 7900GS KO. The extreme edition is just under the XXX-Edition.
So in closing, the eVGA option would be a better choice if you were to choose from
the two that you have linked to. :grinthumb
But just know that there are other versions of the XFX 7900GS that beat the eVGA one.