Buttercream or Ganache under fondant?

Couple things ...

 

In your buttercream recipe, the amount of fat seems high to me.  

 

Ganache vs. buttercream for taste, that's a little subjective.  Personally, I like both, sometimes even mixed together!  However, other factors should come into play.  Does your target like chocolate?  Is the ingredient cost factored in the budget?

 

If your cake is due that soon with that many servings, I would go with a recipe that you have used many times before.  Definitely give ganache a try, but allow yourself plenty of practice time before going into a cake with it.  Ganache can be made into different consistencies and can sometimes be a very different beast between them.

 

Disregard this.  I don't usually use shortening in icing.  I most likely don't know what I'm talking about.

All my fondant covered cakes have white chocolate as the icing. I use white chocolate because chocolate doesn't work with every flavor but white chocolate does (just like buttercream). Also, if I have to take take the fondant of the tier white chocolate doesn't discolor the fondant and I can reuse it.

Fondant can be refrigerated; you just have to be careful about condensation when it comes back to room temp.

Unless the room where you work is the same temperature as your fridge, you'll get condensation! So it doesn't matter if it's winter or summer, or dry or humid, the temperature change is what causes it. Think of pouring cold water into a glass...eventually condensation forms on the outside. It's ok though, just don't touch the fondant until the condensation has evaporated cause you get fingerprints on it.

 

Me personally, I use ganache on all my cakes. I love the way the cakes look and IMO it's easier to apply the fondant perfectly and with nice sharp edges. But definitely practice first, not on this cake your doing now.