[tenpaiyomi]

Third (personal, non-research) opinion based on general observation among myself and peers. Neither approach is a be all end all, some people learn better on one approach versus another, even among different skills.

Personally, when I am approaching something that is a "book learning" type of skill (Mathematics, Science, etc.), Mastery Learning is more efficient for me. When I am learning something that is a "hands on" type of skill (another programming language, metal working, welding, etc.), Spiral Approach is more efficient for me.