for me the best way to compare is just looking at the charts
and benchmarks all around the web,
don't confuse you're self with the numbers
yes they do say about there performance,
but the real performance is in the real world ,
From an analogy on another thread by another person:
Think of nvidia's processors as big trucks, they could do 150mph but carry a ton of load. The ATI's processors as small Supra's that could do 200mph but carry 1/4 the load of the truck. Each has its own strengths but benchmarks and actual usage wins the day.