Should I expose ISubject<T> or IObservable<T> and IObserver<T>?

Question

I have a value that I want users to be able to subscribe to and push to, so internally in my class I'm using Subject<T>.

When exposing this value as a property am I best to expose it as a ISubject<T> or is it better to split ISubject<T> into IObserver<T> and IObservable<T>?

Option 1:

    private readonly ISubject<bool> isLightOn = new Subject<bool>();

    public ISubject<bool> IsLightOn
    {
        get
        {
            return this.isLightOn;
        }
    }

Option 2:

    private readonly ISubject<bool> isLightOn = new Subject<bool>();

    public IObservable<bool> IsLightOnOut
    {
        get
        {
            return this.isLightOn.AsObservable();
        }
    }

    public IObserver<bool> IsLightOnIn
    {
        get
        {
            return this.isLightOn.AsObserver();
        }
    }

Which is better and why? If option 2, any thoughts on the naming convention also welcome.