If for some reason I build both a Cinderhulk enchantment and a Sunfire Cape, which of the two would create the aura?
From what I've read on the forums, it's the one that will do the most damage at that time. Basically Sunfire cape will be on always until you get into a fight around Monsters.
Here's a quote from the Cinderhulk conversation on the wiki page:
You are right, both passives will be active. But they DON'T stack, the game will choose the Immolate aura who will deal most damage. At the beginning of the fight it will choose Sunfire cape damage, but when the Cinderhulk is getting charged up, it will be the cinderhulk immolate passive dealing damage.
This information from: http://leagueoflegends.wikia.com/wiki/Cinderhulk