I'm not sure that one is better as much as it's personal preference. I prefer milk because of the extra flavor, the added fat ..... and because I have this psychological hang-up about using water.
See, my mom was a HORRIBLE cook! She'd make homemade icing by mixing p.sugar and water together and slappin' it on the cake. Just p.sugar and water. No fat, no flavoring, no nothing. Just p.sugar and water.
Nasty. really, really, nasty.
Some ask if I became a good cook because my mom was a good cook? I tell them, "Nope! I became a good cook for survival!"
I prefer using milk. In my opinion it just makes a better icing.
The amouount of liquids added to the recipe I really dont think makes a huge difference in flavor to my opinion unless you are using a creamer that has lots of flavor in it. The
idea of using milk in the buttercream came because of the fact that the vegetable shortenings sold in the supermarkets had ) transfats, so adding whole milk to the buttercream would add some of the fat that the shortening was lacking
Edna 
I use Heavy Whipping Cream
Yep. 
I use milk here as well basically due to taste factors.
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