Ruin 1 vs Ruin 2

[Lovemonster]

If you are DPS'ing in cleric stance as a scholar and have time to use ruin that means there's not much healing involved so you are using energy drain. In this case the MP doesn't really matter.

One nice advantage of using ruin II over ruin I is that you can auto-attack as well. If you are in melee range of the boss (a good example is rafflesia) you can use ruin II to also auto-attack so your "potency" increases. Ruin II gives you opportunities to weave and move and is overall more flexible. It really doesn't matter for a scholar. It does matter for summoners because they have so little MP.

[Eltaire]

Personally, I use Ruin I for the most part.

I'll only use Ruin II if I need to be quickly dodging AoEs, as the MP cost can be quite draining, especially in some of the later dungeons.

I like the benefit of Ruin II blinding the enemies, but it's only for around 5 seconds, so it is only useful in the short term, unless you keep casting it.

There is definitely benefits to both spells, and I certainly made use of Ruin II in solo instances of the story, where I could only rely on myself (I know that there's AIs in most of the story battles, but generally they are pretty bad).

[Gallus]

If you use Ruin as a SCH, I'm sorry, but you are absolutely terrible. There are so many advantages using Ruin II over Ruin as a SCH I don't even know where to start:

1 - Instant cast, no aoe will ever force you to interrupt this spell.
2 - Can autoattack once per Ruin II, meaning every Ruin II will have around a potency of ~100-105 instead of 80.
3 - Can do emergency healing in case someone takes damage without having to interrupt anything. You are just say casting your Ruins II against a boss, tank gets a big hit, ac lustrate <tt>. If you were casting Ruin, you'd have to interrupt the cast and cast lustrate, a lot more trouble.
4 - The 3 reasons I gave above should suffice. The mobility you gain from using Ruin II over Ruin should really just be enough, despite mobility not being the only significant advantage you obtain from using Ruin II over Ruin. What is more, if you are running out of mana with a 4500 SCH because you are using Ruin II, uninstall.