I'm considering getting a 15" MacBook Pro and a store I'm looking at offers two different models -- MA464LL/A and MA601LL/A, which have identical specs but the latter is $500 more expensive. Does anyone know what the differences between these models would be? I think I heard that two different generations of MacBook Pros were released, maybe the latter model is the newer generation? If so, what has been improved between the two generation?