Preformed v rubber liner

What is better a preformed pond liner or just pond liner?
I am building in my back garden, and can't decide what would be better preformed liner or just loose pond liner. Any ideas?

Answer:
You can never get a preformed pond level! And their life span is so very short. You will get a couple of years out of it and it will crack. Use a 40 mil butyl rubber liner. That way you can have the size and design you want and it lasts forever. And you don't ever need shelves. All they do is give you more surface area for algae to grow.