Q. Which is better, goatskin or calfskin?
It depends on which examples of goatskin and calfskin you're talking about. The Nelson calf is a soft, matte-finish calfskin, incredibly flexible. If you read my piece about the Nelson Signature KJV, you'll get more of my thoughts on that. Cambridge calfskin back in the 90s was a very different animal, something supple, sometimes stiff, but never matte and never soft. It had a more structured, traditional look than the dressed down Nelson approach.
A lot depends on whether the leather has a stamped grain. The stamping process apparently uses heat, which results in a stiffer leather, which is why a mid-grain goatskin cover from R. L. Allan's isn't as limp as the highland goatskin (which has natural grain). Allan's bindings will have a more traditional, put-together appearance than the Nelson look.
In terms of longevity, I'm not sure whether there's an advantage choosing calf over goat or vice versa. The differences are probably more in the realm of aesthetics. Because goatskin tends to cost more, people think of it as a step up from calfskin, but I've seen calfskin covers I've preferred to goatskin (and vice versa). There are more factors in play, in other words, than just the type of leather.
I'm not even going to mention the so-called exotics.
Having said this, I'm the first to admit I'm no expert on leather. Also, I'm not a leather snob. Goatskin, calfskin, pigskin -- I like them all. I only turn my nose up at mystery leather, the stuff that doesn't specify its origin. So I have no use for "genuine leather" as a label.
I'd be interested in what other people think about the hierarchy of leather. What do you prefer and why? Have you experienced differences in longevity with one over the others?