I like Dirt 3 more. Unlimited 2 is cool, but not quite as good as I expected. But it does depend on what kind of racing you like more.
Dirt 3 is racing on tracks while Test Drive Unlimited 2 is racing on streets for the most part. Whichever concept you like more is the one you should try out. Although if you don't care about the concept, get Dirt 3. Check out my Review on that game if you want to get a feeling for it :)