Getting a new hvac system and not sure if I should get a singe or 2 phase heater?
What are the benefits? Is the 2 phase more energy efficient and phase one going away?

If this is residential, it will be single phase for 120 or 240 volts.
If this was commercial, you may want to go 3 phase.
I have never seen 2 phase. I have heard the phrase.

You mean stage not phase? A 2 stage unit saves gas, but is more expencive. Most regular gas furnaces are 95% eff now anyway so inless you are going to put out the money for a modulating gas valve unit then I would go with the single stage unit.

2 Phase Power Systems and Two-Phase Electric Solutions

Have you shopped for an AC unit yet. If not I think the only thing that you will be offered is a single phase or a three phase. Unless you live in an area that the electric company has made provisions for three phase you will either have to go with single phase or pay for the Electrie company to bring three phase to your neighborhood, that can be very expensive.

Most residential AC's now are single phase.