Another vote for Flex pipe. Btw we are neighbors I just had a piece crack this year but it is the first issue since owning the pond. This was more due to my level of the pipe for water then material used. I like the look of the pipe better if there is any exposed then the PVC also. I did mix a couple areas with PVC fittings on the Flex pipe which worked like a dream. Overall a lot less seems to worry about.
I'm not worried about the work of hard pipe or ease of flex . My concern was which would fair better at shallower depths with freezing. I plan on running pipes that have to run from one side of the pond to the other in order o reach the pump by having the pipe in the excavation of the pond say in the first shelf but at the 90 degree angle which should help keep the water warmer. I imagine the use of flex will also be better for a little freeze and flex but the flex is not as durable.
under both the first underlayment and the liner fill in the rounds of the pipe so it is one with the wall . inside the excavation but not in the water but on the outside of the liner but at even a foot below grade it should be able to keep the water warm to where freezing shouldn't occur