I've done the MSc in HF at Cranfield and it was excellent, I'd recommend it. I have friends who have done the City MSc and they really rated that course too so I think you have to look at the modules and decide which course interests you most, if you're not interested it's hard to keep the motivation going. If your ambition is to reach Safety Manager and beyond I would recommend looking at an MBA or other business qualification, you'd be surprised how many business techniques overlap with safety ones and an awful lot of systems and methodology in both fields have come from engineering disciplines in the first place. Plus you have the advantage of being able to talk the language of those who hold the purse strings. Other than that I agree with most of the other posts which say that experience in different fields of aviation is at least as important as the qualifications.
Good luck
Ronnie