"Result" is a pretty opaque name: any value or type can be called a "Result", really. "Either" at least hints to the fact that it is either-this-or-that.

"Either" is IMO a nice counterpart to tuple types: one is a generic sum, the other is a generic product. People could complain that a tuple of ints are less descriptive than just using a constructor with a more descriptive name - like "Pair" or "Coordinate", depending on its intended application.

Having a specialized type for error handling makes sense, though.