I would think that it depends upon the circumstances:

If you are an obscure or new author who receives a good review from a trusted member of MR, you will probably see more sales than from the same review from the same person on Amazon. Exposure is the problem in this case, and exposure is hard to gain on a site like Amazon.

In almost any other case, Amazon will likely have the upper hand. More eyes. Reviews from a more diverse readership. Oh, and having the buy now button right next to the review helps too.