They're practically the same machine. The extra cache in theory should give you a tiny speed bump but I wonder if it will be all that noticeable. I'd say not to worry about how fast either one is, because they're so close in performance but look at the other features.
For instance, the 09 models have a seal battery and no expresscard slot. You may want a user replaceable battery or you may want the longer battery life. Additionally with the 15" models, you can now get a matte screen.