DEFINETLY the xfx versions. u are gonna see no noticable improvements with 512mb over 256mb on those cards and the extra core speed(especially being that its the same on both cards) will help nicely in overall performance. so go with the xfx's and be happy. the mem clocks on the xfx's are also clocked 100mhz higher, that will provide a noticable performance boost as well. course for the price of 2 6800gs's u could just get one 7800gt/gtx, x1800xl/xt, x1800gto, or 7900gt. all the cards i listed are gonna outperform TWO 6800gs's by themselves. the x1800xt and the 7900gt will outperform them by leaps and bounds. so my advice would be to get a newer card and then maybe go SLI later instead of wasting money on a older SLI setup.
Morgash