There has been a fair amount of controversy concerning the DX10 release of Call of Juarez. A version of the demo was handed out by AMD at the launch of the HD 2900 XT, which was found to inadequately support nVidia's hardware so results were supposedly skued in AMDs favour. Since then there has been ongoing debate about software tweaks crippling nVidia's hardware to show AMD in a favourable light. So, with this in mind, one perhaps shouldn't read too much into the comparative results of nVidia's and AMD's cards and instead just take each card as an isolated result.
There are three key features that have been added to the DX10 version of the game. Geometry shaders have been used to simulate water particle effects like the spray coming off a waterfall, alpha-to-coverage (or transparency anti-aliasing to me and you) is used to smooth the edges of grass and leaves on trees, and a special anti-aliasing technique is used to create HDR correct AA where high contrast edges are involved.
Overall the demo does look stunning but without a DX9 version of the demo to compare to it is difficult to judge how much different the game is with these additions.