What is the difference between phototriac and photocoupler isolation for the G3NA Solid-state Relay?

The applicable output load voltage range is different.

Solid-state Relays with a zero cross function using a photocoupler can operate with a small input current because they have very good coupler transmission efficiency.

Photocoupler Input Current < Phototriac Input Current

The input current for a phototriac is greater than that for a photocoupler, but that doesn't necessarily mean it must be a selection criteria for the type of Solid-state Relays. Other parameters can be used as a basis for selection.