Taxi, as it is not particularly far away, and is not that close to stations. I would just cab it from the hotel, but it may save a bit of money if you take the Ginza line to Shibuya, and then cab from there to Shin in Nishi-Azabu.

Either way, get a detailed map from the concierge. An advantage to getting a cab at the hotel would be that they can talk with the taxi driver to make sure he takes you to the right place. It is easy to get to, but could be very difficult to find for the first time without a map and a taxi with GPS.