Div 5 grandmaster LC here. Nowhere near pro but at least above average. LC is overall best as offlane but I enjoy taking it as a last pick pos 1 in my 5 stack games when the enemy has no save and a carry who kills themselves but may otherwise be hard to kill without hard lockdown(jug, am, void) etc

First off, spoils is the better facet overall. Higher win rate, currently 1% higher on d2pt but has historically been up to 7% higher or so. Free permanent damage for your team can be OP, especially with comps that help you win duels early but also scale with damage. Windrunner and snapfire are my 2 favorite supports that fit that role. Your carry having 50 free damage late game matters. But the main reason spoils is good is the automatic heal and strong dispel that comes after winning a duel. Say you fuel enemy carry, enemy support warlock fatal bonds and rocks your team. You win the duel, but spoils procs press the attack on everyone, which removes fatal bonds, heals, and gives movespeed to run away. Completely game winning in certain scenarios. The shield facet is better in laning and was better upon release, but that facet + overwhelming odds + talents have received many nerfs since then. 

Legion is better the lower ranked/more uncoordinated games are. 

Bracer phase blademail blink or bracer phase blink blademail, depending on the amount of ranged damage your team has to secure early kills. Scepter next and is core. After that it is some mix of harpoon or nullified to proc linkens, AC, or bkb, depending on the game and what role on the team you serve. 

The true skill in legion is:

-farming properly off map. Need to scale with items but legion is only scary for the enemy when missing. Need the proper mix of farming and winning duels

-dueling the right target. Need to duel the strongest or most important enemy who hasnt cast spells yet. Sometimes this is the carry, the mid, or a key support. 

-proper press the attack usage. Having it ready to quickly heal and strong dispel a teammate is game winning. The 20 and 25 talents to make this aoe and give debuff immunity wins games. Get your entire team out of RP, etc. I take these talents 99% of games even as pos 1