Grass fed butter is far more nutritious with many fat soluble vitamins vital for strong immunity, therefore, great health with very important hormonal balance that is crucial for optimum body preformance can only be accomplished with nutrient dense food. Grass fed butter is definitely better choice. Coconut oil has lauric acid which is great for immunity, but butter simply has many other great 'things'.
Why choose when you can have both!? I love both but if I had to choose then of course the fat from mothers milk will trump just about any other fat on the planet.
I can't put my finger on what I like more about each of them apart from the obvious - butterfat has a much better flavour.
A coconut oil stir fry does have something about it that makes it so damn satisfying, yet butterfat has very similar qualities in that regard, I feel like i'm absorbing much more of my food when I use either.
For energy some say you'll get a bigger boost from coconut oil due to the MCT's, but butter also has MCFA's & the SCFA's in butterfat act in a similar fashion, maybe even quicker. I know if i'm in ketosis and have a spoon of butterfat I get energy from it faster than coconut oil.
I actually prefer coconut milk to oil, its still high fat but is very nutritious and very easy to make at home from whole coconuts, there is nothing like fresh coconut milk.
At the end of the day butter wins simply because it has the best flavour & the fat solubles A, D, E & K.
I do have the same environmental concerns about coconut oil. It has a big carbon footprint and as CO grows more popular there's more potential that primal rain forest will be clear cut to grow coconuts. OTOH, grass fed butter doesn't have some of the components that make CO healthful, including a high percentage of MCT's. I use both, but lots of our European ancestors got along fine without ANY CO oil ever, so vote your conscience. Nothing bad will happen if you choose to avoid CO. And Chronic is right, if you choose CO, that doesn't mean you can't also do butter or ghee, and support the local economy and environment as well.