The 360 version does not have inferior textures.
To put speculation about Rage's performance to rest, GI asked Carmack about his work on all 3 versions (which they thought looked comparable, sorry no Mac version was previewed).
The full demo walkthrough was on PC with an Xbox 360 controller, because the game is (cue alarm sounds) being designed around controllers, not kb&mouse.
The PC version will have higher resolutions available, more AA, and faster "page-in times" for textures.
The PS3 version (according to GI) seemed to have slight color saturation tuning needed, and according to Carmack it took more time early on to get their heads around designing for it, and it has more "theoretical raw performance".
All he mentions about the 360 version is that it was easier to get to where they wanted to get than it was on PS3.
The Mac version is about a week behind the other builds and was not shown.