Arc is deterministic, you get the same results on multiple runs (minus not synchronized async fun of course), you can profile it etc. – you don't have this luxury with gc.
Of course, what is deterministic is deterministic, if you use allocations based on nondeterministic random, then you can't see deterministic allocations - I think this is obvious and doesn't have be to spelled out.
With gc deterministic runtime creates nondeterministic deallocations.
With arc deterministic runtime creates deterministic deallocations - reproducible behavior that can be profiled and allows you to work on optimizing it.
