Class vs Struct - Which is best for Shared Instances?

Question

Since, I have started using struct, I am wondering which is best among Class and Struct for creating Shared Instances.

Class Example:

class Helper{
    func isNetworkReachable(){
        return reachability.isReachable
    }
}

Usage:

//Below taking as a Global Instance
let helperInstance = Helper()
print(helperInstance.isNetworkReachable())

In Class, as we can see that shared instance is created and it stays is memory till application terminate.

struct Example:

struct Helper{
    static func isNetworkReachable(){
        return reachability.isReachable
    }
}

Usage:

print(Helper.isNetworkReachable())

In struct, the static keyword plays the main role as it also specifies that the instance will stays in memory till application terminates.

In short, I want to know which is best, Class or struct and why?