So the single type of mobs I'd say I hate most is fiends. They bug me, so much. Anyway, a while back I happened to pick up a random triple UV swift flourish. CTR low, undead med, fiend med. Payed 900 CE for it. I run mostly in divine skolver combo which gives me fiend med and dmg med. Combined with my focus trinket, I would end up getting max damage and ultra ctr on the flourish against fiends. I think I could probably sell it for a decent profit though, and I've been wondering if an ASI flourish would be more effective for fighting fiends. I can't even use the charge attack much of the time because of all the flying pitchforks. On the other hand, the charge works wonderfully on greavers, but I could always grab a ctr med for that and get full ctr.
Soz for the long post, and for kinda rambling.
As you said, CTR is not that great, since you are not going to charge it against devilites.
On the other hand, ASI lets you shield faster after attacking, on top of the obvious ASI. Damage is generally not the biggest issue with fiends : not being hit is.
So I would go for ASI. The thing tho, is FF is already a fast sword. You are probably not going to notice the speed difference. Which can be infuriating for you, because ASI UV on Flourish is expensive.
Well, no clear answer, it's up to you.