The first is static field; it's not even a property.
Static fields have only one value for the application.
While the second is an instance property, which is different for every instance of the class.
It does not make a big difference in this example, if it is a property or field. But in the long term, if you use a property in your interface, you might change it later to have an actual getter and setter that do for example, validations or make the object react somehow on the new value. A field is just a field, you cannot control when and how it is set and react to it.
