I think you can buy either one (the cheapest) and then download the Black Edition patch for free. I have the normal version, but I've never downloaded the BE patch. I think it adds a few cars and races. Maybe some other stuff, but nothing major.
You may find the patch at: http://www.nfsunlimited.net/mostwanted/utils/
If it doesn't work, you may try the official ea.com website or the External Links section in the Wikipedia article.
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