Being a flamethrower, the Ignis and Ignis Wraith both start off dealing only Heat Damage before you start modding. This makes them particularly good against Infested units, but depending on how you build it can be effective against some Grineer and Corpus. Also note that before modding, Corpus units with Proto Shields will have resistance to its Heat Damage. Unlike other weapon variants, there is not a notable difference between the two as far as secondary abilities, different effects, etc. Overall though, the Ignis Wraith is a stronger version of the Ignis pretty much across the board as shown above.
Modding the flamethrowers for Range is definitely a plus, as both have innate Infinite punchthrough against enemy units. Add some punchthrough mods and you can also hit enemies behind cover and even through doors. A Range and punchthrough combo, as a note, also works well against Kela De Thaym’s Arena Wall Targets.
Both variants fire a beam 0.15m wide, giving them the widest beam of all continuous fire weapons. At max Range or when the Beam hits an impenetrable surface, damage is dealt out in a spherical radius of 3m. There is a bit of a ramp up on their Damage, starting at 35% when you first pull the Trigger. Then after 0.6s you’ll be at 100%, which then decays back to 35% over 2s after you release the Trigger.