I believe the PVC is thicker and copes better with exposure to sunlight. The polythene tends to break down over time.
The proper liners seem to vary a lot in price. If the pond is relatively small, which yours is, I doubt you need the high end thickest stuff. Bear in mind it also is very heavy so dragging a large piece into position isn't easy!
If you aren't on a tight budget and want to do the job right I would go for the pond liner. I think with the liner you will have less need for an underlay as the liner itself will absorb the lumps and bumps.