Llum is a step back from the OP insanity of Samael, thankfully, but not much. An outstanding monster.
Unlike some Nemesis, the choice of variant is more a matter of preference than anything else.
Like Hydratila, the vanilla cheaper version is just as valid a choice as the expensive models.
This is because her core defining skills are common to all variants.
(As an aside, remember that even an unranked Nemesis can access G4 skills by taking all G3 skills; the next time a G3 skill would be offered a G4 skill will be available).
Battlefield Speech is of great tactical value and essential imo. Double damage and precision is key to unlocking defences. Throw in the remove negative effects along with it, and this is an outstanding skill. Note that the double damage says it lasts 1 turn but in fact lasts 2 turns.
Accepted Fate is another exceptional skill. 50pt neutral aoe together with Blind and Damage Reduction and stripping positive effects makes this a move that would find an unhesitating skillset home on attackers, deniers, or support monsters; on attack or defence.
Both of those skills are no brainers. Those two skills alone put her among the best of the attackers, support, and defensive (soft) deniers (do you want to be blind and damage reduced on attack?).
All versions of Llum then have two skills which continue the trend of reducing Thetys' prior dominance: Divine Mission (0 cost 0 CD all team energy regen) and Real Rumours (0 cost 0 CD aoe that does no damage but has 50% chance of inflicting each of blind, damage reduction, and stamina drain - yes, 50%, but three separate checks).
Both of those are valid skills particularly on defence; the latter helps negate Timerion too.
All 3 variants have Leader Shout, which again strips all negative effects on your team - while healing them all 30% and giving them damage buff. This synergises well with her Battlefield Speech, giving your whole team triple damage (+50% and double damage stack in geometrical progression giving triple damage) and precision. With Samael or Demise, this combo is entirely feasible and allows for very fast speed runed lethal attack teams with few or even no strength runes.
After all that, what does it matter which version you run?
The cheapest version with Light weapon (Light of Freedom) is a good bet for killing those nasty Samael, since he is with her in Spirits and that 65pt Light attack crit hits him hard even if the Sunburn doesn't stick (in 130 v 130 combat, she will ohk Samael with 1 strength rune with either 50% boost or double damage, not hard to engineer with Demise or Samael).
Her 45pt Light aoe + Sunburn is also daunting soft denial on defence to complement Accepted Fate (do you want to risk an attack team running with -15% accuracy, let alone coupled with Blind and Damage Reduction?).
The metal weapon version (Iron Leader) is an outstanding attacker. 70pt metal damage with bleed is up there with the very best. 50pt aoe metal and bleed means she has two 50pt aoes! Moltus, you are obsolete.
The Magic version (Magical Matriarch) has the much touted stun denial in single and aoe flavours. I am not greatly impressed by stuns. In many rules they are easily overcome or ignored when on defence - Goldfield is also in both Spirits and Evil Legions. And opening with a 90% accuracy denial on attack is unpalatable.
And her 0 CD magic attack, which could otherwise have swayed me for Timerion killing duties, is only a 35pt attack with a 50% stun (which won't bother Tim in the slightest).
So on balance, the Magical Matriach despite her superior speed is my least favourite of the three.
Bottom line for me is that any variant is exceptional and her versatility is outstanding without compromise.
I took the Iron Leader, despite having several good Light attackers like Lau Lau and Valgar, primarily to kill other Llum before they disrupt my attack plans. But there is no reason not to just settle for the cheaper Light of Freedom.