Originally Posted by Drip

I have LCs, but still won't spend them on these buffs unless that would leave me with enough LCs left to buy at least the Rainbow Skillbloom and the 2 150LC Flowering Hearts next month.
It's fairly probable that I'll be in your exact situation next month though, and then I'd probably go for either the recovery boost (but I want to know how that works first) or the Critical Up.
DEF activation up might be nice against raidbosses though, since those fights tend to last longer.

Mousing over the abilities in the "Edit Ability" tab gives more info: Recovery boost increases healing effects on the character by 25% and overhealing is added to a max 5% health cap boost (if your girl was at full health with a value of 20k, the extra healing would give her up to 21k max health.)

As for the OP, I'd choose recovery boost. My Alpinia with lv 5 skill, red necklace/earring and extra healing to boot? No question.

Critical boost only gives an extra 5% crit rate and DEF up increase defense activation by 2.25% and damage reduction rate increases by 2.5%. Meh.