This is a general question that applies to any class hierarchy, not just repositories. From a pure OO point of view, an interface and a pure abstract class are the same.
If your class is part of a public API, the primary advantage of using an abstract class is that you can add methods in the future with little risk of breaking existing implementations.
Some people also like to define an interface as "something that a class can do" and a base class as "what a class is", and therefore will only use interfaces for peripheral capabilities and always define the primary function (eg. repository) as a class. I'm not sure where I stand on this.
To answer your question, I don't think there is any advantage to using an interface when it defines the primary function of the class.
