So the event guide page shows 42 baking chocolate in the 35 AP four cavalries mission, and 30 class coins, versus 40 baking chocolate and 32 coins in the 40 AP four cavalries. On the other hand the three knights score much lower in both baking chocolate and class coins.
It looks to me like the most efficient path, assuming you want an event CE for most classes, would be to grind the 35AP four cavalries for a while to max early grinding towards the melty sweetheart CE, and then once you start getting all the CE for four cavalries servants, swap over to three knights? Alternatively just focus down whatever you need most once you get all four melty sweetheart CEs?
Once you have 4 melty sweetheart CEs and all the valentine CEs you want from your waifus, you should focus on either the 40 or 35 cavalary quest. The difference is minimal in terms of AP per coin/chocolate, and boils down to whether you want pages (35 node only has books+demon!) or gears/pages/teeth/crystals (depending on the currently active enemy rate-up)
