I had Frostfang while on Zilean, and it is supposed to proc by using spells and basic attacks. It was proc'ing with basics, however when I used my E, it did not give the 10 gold. I tried this multiple times throughout the game and it did not work.
Has to do damage to an enemy with the spell to proc the gold. Not just target with the slow.
Frostfang on Zilean's less effective because you only have 1 damaging spell, plus that spell has a pretty darn big AoE... 1 minion dies and you can't get gold for a while (on the first two levels of the item). Personally, I'd go with coin because mana good... AoE speed boost is fun. Last hitting a minion is less gold-efficient overall than letting a lane mate take it, but you don't lose as much.
