Given the time developers will have had with the 360 I don’t think you'll be able to tell much difference between the two platforms.
Lets say hypothetically Sony show Gran tursimo 5 and Microsoft show Foza 2. I don’t think they will look too different in graphic terms; art direction, AI, handling and physics will differentiate.
Although Sony will need a excellent online mode seeing as Xbox Live is a given.
Microsoft’s experience in that field may pay dividends, although Nintendo managed a ok first full online game in Mario Kart DS.