In terms of DPS, the Charizard will be superior choice but if you can TM the Moltres to Overheat then they'll be roughly equal. However, if TDO is considered then Moltres will win out regardless.
Honestly though, Fire types don't really any relevance at the moment. I'd save your resources for a time when they do. Charizard in particular I don't think is worth spending rare candies on (and this is coming from someone for who it's a personal favourite) - another nest or event and you could be swimming in Charmander candies. Moltres is much worthier in that regard.
