You shouldn't bother with Smackdown... unless you really want to be able to say that you've played through both brands. The bulk of the reason why I got a refund was simply because there was nothing different in the storylines. As you've said, it really is a case of "Same ****, different day."
Even the dialogue is more or less exactly the same. I don't know if it would unlock anything different... but I just couldn't stand to play exactly the same game; especially one as annoying as this.