Answer: Both types can work well and are very popular. Durability and expected life of the specific liner or tub would be noted by the manufacturer, there is a range of quality between the different types of plastics and products on the market.
To some extent your choice will depend on how large a pond and how deep a pond you want. Many of the preformed ponds are small and relatively shallow, and you are limited to the shapes they offer. They can be more easily leveled, I think, than the free form liners. With a soft liner you can custom size and custom shape your pond and go down about three feet which is what you would want to overwinter fish in it.
In either case you need to carefully read the installation instructions and follow them fully to maximize the life of the liner or tub. Sharp stones and in some cases sunlight for example can contribute to the material deteriorating so you may need to lay down a protective pre-liner and also line the excavation with sand, then cover the lip of the pond with stones to block the sunlight from hitting it. Ice freezing in the winter can potentially cause problems; you will also have to maintain your pump and possibly also a filtration system to avoid clogging the pump for your waterfall.
Some people will combine both, using a free form liner for the pond and a preformed waterfall. The preformed waterfalls are great because you know they will predictably run the water and they are quick and easy to install. On the other hand a soft liner will allow you to arrange your waterfall, either formal or informal, in whatever way looks and sounds right to you and/or to custom fit it to your site.
I hope this helps you plan your water feature!
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