I don't know Swift very well, but I seem to recall the creator talking about using automatic reference counting instead of garbage collection as an intentional tradeoff. If that's still the case, I'd much rather use Swift over C# in a game (and I am using C# in a game!). The value types and structs are very limited compared to classes and reference types.
Yes ARC is definitely an intentional and well-chosen tradeoff. Swift has real first-class value types (strings, collections, etc are all value types) and copy-on-write semantics that make it all work quite well. C# value types feel like they are funkily bolted on and aren't really very useful in practice (at least in Unity).
