The most important difference on OS X is that ARC is compatible with retain/release, but GC requires explicit support by every framework linked in your app. Some frameworks never added GC compatibility.
In the general case, it is estimated that GC uses on average twice as much memory (high water mark), but direct comparisons are of course very application specific. GC also has the dreaded collection pauses which can make performance unreliable. (Eg stuttering animations)
Biggest drawback of ARC is that it doesn't detect cycles, so the developer needs to have a global understanding of an app to avoid hard to debug leaks.
In the general case, it is estimated that GC uses on average twice as much memory (high water mark), but direct comparisons are of course very application specific. GC also has the dreaded collection pauses which can make performance unreliable. (Eg stuttering animations)
Biggest drawback of ARC is that it doesn't detect cycles, so the developer needs to have a global understanding of an app to avoid hard to debug leaks.