Halliday & Matthiessen (2014: 318):
Means is close to the participant rôle of Agent and Comparison is like a participant in a clause with the same type of process, whereas Quality and Degree are like features of the process itself. These differences in status are reflected in realisational tendencies: Means and Comparison tend to be realised by prepositional phrases, whereas Quality and [Degree] tend to be realised by adverbial groups.