I live in the northeast and was wondering if anyone in this climate has
installed a heater for an above ground pool. I was wondering what
benefits each one has. I am looking to use the heater during the early
and later parts of summer when the nights get to be quite cool. Any
thoughts are appreciated.

Here are my thoughts on your question.
1) Which source of power is cheaper? gas or electric?
2) Heat pumps are a great alternative for heating and cooling homes in
moderate climates. May not make sense for just heating.
3) Heat pumps are basically compressors. Lots of mechanical parts. I
would stick my neck out that a gas-fired furnance will last longer than
a heat pump, and replacement costs to be less expensive.