In regards to this question "What factors contribute towards reliability in your opinion?" I would say the manufacture actually making the heater we have dealt with all of the major heater manufactures and we have had very little warranty issues with Raypak and Hayward. I tend to like Raypak a little more as that is all they do is pool heaters and nothing else so they are very focused on this product line.

As far as specific features for reliability you may consider a Cupro Nickel heat exchanger. The main component of a gas heater and by far the most expensive part if it needs to be replaced is the heat exchanger. Most heaters come standard with a copper heat exchanger however you can typically upgrade for a few hundred dollars more to a Cupro Nickel heat exchanger which is a heavy duty heat exchanger which holds up better against your pools chemically treated water, this is a good option if you have a salt pool as well.