Multi-platform titles are not fully or not equally optimised for all supported game systems. For this the XBox 360 and PS3 are far too different.
Top PS3 exclusives usually have better anti-aliasing than XBox 360 exclusives.
A good example would be comparing top high profile, top high budget exclusives like Halo 3 and Uncharted 2. Uncharted 2 has anti-aliasing and Halo 3 does not, in addition Uncharted 2 provides far more detailed and varied environments, steadier framerate, better lighting, better physics, better audio and overall more seamless experience.
And of course Halo 3 isn't even rendering in high definition!
The EDRam on the Xenos daughter chip could have been an important advantage equalling things out in terms of graphics potential to some extend (not with regard to game complexity though and Blu-Ray/default harddrive would still be a PS3 advantage with regard to streaming). But sadly the amount of EDRam is too small for high definition gaming to prevent technical sacrifices due to tiling issues.