Both HDPE and EPDM are very heavy, but HDPE is significantly heavier and thicker. HDPE is considered a heavy-duty liner and while it can theoretically last a hundred years or more, it has some very significant disadvantages, including that it is vulnerable to significant damage from weathring, tolerates temperature fluctuations poorly, and is difficult to bond. It's also highly flammable and liable to crack when exposed to stresses over time.
EPDM, in contrast, is regularly used by homeowners building small backyard ponds as well as large decorative installations for commercial properties. It has excellent resistance to weathering and tolerates a wide range of temperatures, and both repairs and joining seams aren't much more complex than repairing a bicycle tire. Under optimal conditions, EPDM could have a life as long as 60 years or more, though warranties typically don't extend beyond 20. Nevertheless, for most practical purposes, EPDM turns out to be a better choice.
