I've not done, or taught on either of these courses, although I know both institutions.
In my opinion, they are of significant value to a mid career airline pilot looking to move into management roles within the industry, either instead or whilst continuing to fly. They are probably of little use to anybody else, although I've known people who aren't pilots trying to use these courses to pursue other career routes - and best of luck to them.
Also in my opinion, both are good Universities, but Cranfield: for aviation masters degrees, is world leading (compared to City which is only world class). It's also in a nice bit of countryside instead of a big expensive connurbation.
An FI rating I'd have thought is quite a bit quicker and cheaper, and quite a different beast. It certainly won't eat a full year of your life, which an MSc certainly will, it'll also bring short term financial return - but won't build a long term career to senior levels in the same way that the MSc might.