Therm is more offensive, HE means your opponent will have to manage their blue bar much better. Melt Armor from range is nicer than Anguishing Cry.
Sonic is more defensive with an impressive passive Regen and world of Pain you will be more survivable.
That being said, I'd much rather Fight the Pain Dom in a 1 v 1 than a Therm.
Which 1 is better for arena/ 1 v 1 pvp and why?