Take a look at both and see what kind of syntax sets you use and the skills of your team.
Rhino Mocks has been around for 4 years and supports a couple of different symmatics. Recording / playback is the oldest, and Ass Arrange Act Assert (AAA) is the newest. I would say that in the Rhino community, Mocks is a standard structure because of how long it has been around and how easy it is to find examples, documentation, and help.
Moq is valid for about 1 year, and relies heavily on .Net 3.5 syntax like lambdas. I think that because of how new this is, it can be harder to find people who have used it and get help.
More important than the structure you choose is an understanding of how layouts help you test, as well as how they can change the way you think about developing systems to test. After all, structure does not matter; experience and knowledge matter.
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