To the OP, sorry if I arrive a bit late.
I doubt people will not recommend their instructor
I would do it another way: what are you looking for in an instructor?
Some instructors teach in a very structured fashion, which is very good for some people; some are not structured but flexible, which may be better for other kinds of people.
Some instructors love to answer many questions, and if you have too many, they will go together with the class for dinner, answering all the doubts not strictly related to the course. Others may prefer not to go too off-topic to avoid confusion among their students.
Some have tons of experience with specific subjects, and some with others.
Lastly, it's a holiday, so if you want, you should consider other factors such as location, food, logistics, etc.
NOTE: there is always a basic structure behind any GUE course, as any instructor will tell you - so "flexible" vs "more structured" is not something like "a mess" vs "rigid": both approaches follow a basic structure and instructors ALWAYS give students a dose of flexibility. It's just that some instructors will stay more on one side of the spectrum or the other.
About Mario Arena: crystal clear and straight to the point when teaching, relatively flexible in managing the course, he loves discussing ANYTHING related to diving and will literally answer any question you have. He has infinite practical examples to discuss decompression issues (I asked TONS of questions).