Macworld recently compared the most recent four and eight core machines. Their conclusion was that for everyday use, the four cored machines performed faster than the eight cored ones. The eight cored machines showed a difference only if you were using some high powered software (like, say FCP) that could specifically take advantage of multiple cores.
OSX 10.6, with it's grand central architecture, may make a big difference in the two machines once it's released, but that's not for certain. Unless you're working at skywalker ranch, I'd wait until snow leopard is released before I bought either of the systems.