RE: what engine you invest you money?
For my money it will depend on what boat and how I intend on running her. There are boats where a QD is probably a good choice - there are others where (in my opinion) a Zenoah might be preferable.
I like QD's but am past the point of the whole QD's are #1 Rah Rah. Sure they are good motors - but there are others that are as good (if not better) in some situations.