It does really depend on the type of pond and if you have stones or tiles morterd on top of the edge.
We really will need to see pictures or a detailed desctiption of the construction to determine the best solution.
For instance, if the edging has cemented on finishing stones or masonry then doing a liner would require removing this and redoing it after the liner is in place. Otherwise you are asking for trouble. Been there, done that.
We need to know the actual dimensions. If it is small and has an edging, then something like Pond Armor would be the best bet. Also been there, thread to appear soon.