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.

preformed v. liner ponds

I live in Memphis, TN. I am considering putting a
pond in my back yard---possibly this weekend if I get real energetic! 
My question is this, my neighbors across the street purchased a preformed
pond, another friend of mine has a liner type---do you know if one is better
 than the other?

Pondlady sez:

 Use a flexible liner. Never use a preformed pond unless you have no choice. It is a major hassle and will never look right because you cannot get it level. If you do use a preformed pond, only dig it in to the shelves and either put soil to the top or pile rocks to the top. Cantilever rocks over the top so you hide any trace of the plastic. Plant around it to soften the hardness of all those rocks.