I've seen many other people recommend coconut oil ahead of butter. Apparently it interferes less with glucose metabolism, may improve health and efficiency of the liver etc.. Would there be any reason to favor butter (or other reasons to favor coconut oil) ? It does taste better so that's one..
If you can get very high quality grass fed butter, I'd say that it's better than CO in many ways. I get mine locally from a farmer I trust. But when comparing CO to standard supermarket butter I would probably opt for coconut oil. People say CO helps with transit time more than butter, but I've found butter to keep me regular far better than CO. Also, CO has much more potential to become allergenic, butter does not have this concern.