Which design is prefereble? Derive from a subclass and override an abstract method or derive from same class and overload exactly one of 2 possible virtual methods?

dual.cs

public abstract class Command<M> where M : Model
{
   public virtual void Execute(M modelIn, out M modelOut)
   {
      modelOut = null;
      throw new NotImplementedException();
   }

   public virtual void Execute(M Model)
   {
      throw new NotImplementedException();
   }
}

public abstract class Command<M,R> where M : Model
{
   public virtual R Execute(M modelIn, out M modelOut)
   {
      modelOut = null;
      throw new NotImplementedException();
   }

   public virtual R Execute(M Model)
   {
      throw new NotImplementedException();
   }

}

subclassed.cs

public abstract class ImmutabilityCommand<M,R> : Command<M,R> where M:Model
{
   public abstract R Execute(M model, out M modelNext);
}

public abstract class ImmutabilityCommand<M> : Command<M> where M:Model
{
   public abstract void Execute(M model, out M nextModel);
}