It might have been different in earlier events, but for this event
-Small Cornucopia gives best returns on Daily Prizes and SunFlares (and thus Grand Prizes) (and if better of the three on non-double, of course still better of the three on double days)
-Large Cornucopia gives best returns on Knowledge Points
-Medium Cornucopia gives best returns for most of the other stuff
It's deceptive, in that you need to figure in the odds of getting additional Moon Shards, and thus being able to open more Cornucopias. Think of it as the Fantasy World version of the sci-fi "deposit a dollar in a savings account and time travel into the future" compound interest scenario. Or "And for my third wish, I would like three more wishes."
And yes, it really helps/is necessary to save up Moon Shards for Double Days.
A goal of 490 Moon Shards, huh?
Lets say 1 Moon Shard/hour over 19 days remaining = 456 MS, 918 with the Moon Mirror, 19 days of 35-MS quests = 665 MS, for a total of 1,583 MS. That would be 11 more LCs, giving 220 SFs direct, or 31 more SCs, giving 155 SFs direct. Do all of that on double days, and you're at 440 or 310 SF. Factor in some compound interest from getting MSs back, and it's not impossible to get 490 SFs by the end.
Factor in to that, though, that the Grand Prizes are set pieces, and the other set pieces might not be offered on double days, so you might not get the complete set(s).
Good Luck.