Molecular weight of ammonia = 17
Molecular weight of R12 = 121

The problem is that the ISS ultimately rejects heat by radiating it away through radiators mounted on the solar array wings - and water would freeze and plug up the radiators.
 
So instead, they use a water loop to cool the atmosphere and equipment, and then transfer that heat to the ammonia system which then circulates through the radiators. (It's pretty easy to design the system such that there is minimal ammonia piping (and thus a minimal chance of an accident) inside the manned spaces.) Since ammonia freezes at a much colder temperature than water, this means it's much easier to keep the coolant moving at a rate where it radiates enough heat to be useful but still stays warm enough to not freeze.
 
It's going to be a complex trade off to choose a coolant, and few people seem to realize that NASA does take into consideration cost and availability when making their choices. They aren't so good at controlling costs as they might be, which is understandable since overall they're working at the bleeding edge of engineering, but that doesn't mean they don't try.
 
And really, there's isn't much of a difference between ammonia or anything more exotic because even something 'safe' (like nitrogen for example) is still going to rapidly displace the oxygen from the air (that is, reduce the effective partial pressure) if there's a leak because of the small volume of the breathable atmosphere.

Reason 3 which I forgot to include, is radiation turns ammonia into harmless H2 and N2. Little to no effect on the equipment or thermodynamic properties at any reasonable concentration. If you catch enough gammas to break down 50% of the refrigerant, roughly 50% of the crew mass would have been broken down, indicating bigger problems.

Radiation turns fluorocarbons into fluorine and assorted debris. fluorine, at any concentration, is not good in anything except fluorine tanks. Anthropomorphizing it a bit, F likes to halogenate hydrocarbons like pump oil or plain ole oily contaminants leading to all kinds of entertainment. Its just nasty stuff even at the lowest concentrations. I suppose you could design and install a nice heavy halogen trap, but you'd never Really Know about the internal corrosion levels of the pumps and pipes without very expensive continuous maintenance. The entire refrigeration system would need to be halogen compatible. On earth its not an issue due to low radiation levels and frankly if my A/C pipes corrode out its not life threatening anyway. But not so good of an idea in space.

Finally ammonia is high temperature stable and if you somehow manage to dissociate it anyway, the N2 and H2 are mostly harmless and can be flushed out. On the other hand, SOME of the fluorocarbons have pretty nasty icky byproducts if you overheat them, by, say, the pump shutting down in full sunlight for a long time.

In terms of performance, ammonia is one of the best refrigerants in vapor-compression systems. Freon is easier and safer for small systems but ammonia is preferred for large industrial applications.