ForkedLightning - the main skill, deals x2 dmg in close range.
Thunderstone - vs bosses, skip upgrade if you don't have relic
Tempest - good vs crowd of mobs, but when there only 1-2 mobs on screen and it realeases only 1 bolt, it deals literally 5 times less dmg than Forked, so only useful when you come owerpowered in location. Like lvl140 in dunc.
Superposition - useless skill. Even without reduced healing it is questionable, because there is not so many situations, when your standart 3s tele from charm is not enough. But reduced healing just kills this skill.
ChaoticSpark - oskill (from sorc body armor), deals slightly more dmg, than Forked, but aoe is much better. It is little bugged for sorcs right now, but maybe will be fixed in future. If it will be fixed, hugely recommed to switch on it.
Also it is very cool to get 1pt in miasma + upgrade vs bosses, because it slows targets and you can easily kill them with Thunderstone
In comparison to fire sorc, main drawback of light sorc - she is semi melee caster. ForkedLightning goes forward quite slow, range is medium, but dmg at max range is quite low, while at close range dmg is huge. Flamefront moves much faster and travels much further, but doesn't pierce enemies.
Thunderstone also melee spell, because it misses 90% times vs fast/teleporting enemies at medium+ range. So you need to come closer. Flamestrike don't care about distance and can quite reliably hit enemies far away, but deal slightly less dmg.