True, I don't expect any huge differences, but if one machine was particularly better at shading and texturing wouldn't that enable more advanced effects?
With MS owning DirectX, I'm pretty sure they are allowing ATi to put more advanced(unreleased) features into Xenos that RSX won't have...
Yeah, the texturing on MGS4 is horrible IMO. Check out these screens and look at the wall and the tank:
http://img.gamespot.com/gamespot/images/2005/257/reviews/926596_20050916_screen005.jpg
http://img.gamespot.com/gamespot/images/2005/257/reviews/926596_20050916_screen006.jpg
http://img.gamespot.com/gamespot/images/2005/257/reviews/926596_20050916_screen004.jpg
Contrast those with the superior textures in Kameo and Gears Of War.
This is not a knock on MGS4 because I think the smoke, lighting and character models are excellent. Snake in MGS4 has more polygons than any other character I've seen for Next-gen so far. It appears to be a trade off of sorts.
One thing to think about though is a high-polygon model is immediately noticable on either high-def or Standard Def TV. SDTV users will be very happy with MGS4's graphics because they don't have the resolution to see the low-res textures. So that might be why Konami went that route, but the low-res textures might turn a few HDTV owners off. Now with the more detailed and advanced texture techniques(like what we see in GOW and Kameo), it takes an HDTV to truly appreciate them. So HDTV owners would be very happy, but it would be harder for SDTV owners to distinguish any differences from this gen.