The majority of the occassion where multiplat games look better on the 360 is due to lazy coding. The PS3 is more difficult to code than the 360 (a major advantage to the later) which tends to get developer into the 'good enough' mentality.
When you are working on a deadline and coding for one console is significantly more difficult than the other, chances are that there will be less polish on the harder to code console.
Multi-plat games do not show the true capability of any console as the games has to be restricted to an architect that both consoles can handle.
The true power of a system has always been highlight by its exclusives, these games can take advantage of every single feature a system has to offer without having to worry about cross-platform conflict.
Both system have great potential, looking at games like Gears of War 2 and Killzone 2 will show you that.
Time will tell which can be maximised further.
