It turns out Date Trees require just 2 Water, not 5 as one website used to state. Hence in a Grove, each tree requires just 1 Water. Selling Dates directly – without even crafting – yields a humongous $2568 / Water, over four times higher than the next best fruit, Plums. They also give a total of 39 Exp / Water, which is higher than any other tree or land crop, and higher than nearly all recipes other than animal products or river crops.
This does answer the question of which is more rare out of coins or Exp: Exp is rarer. Specifically, you could max out your expansions not too long after unlocking Date Trees at level 59, and well before the level cap of 80.
This exhorbitant coin profit does make the game more boring. However collecting Exp is still a challenge, so keep feeding those Prized animal barns, etc. I’m also now focusing on other challenges such as achieving full Mastery for crops and animals.