I'd say wait if you want, could always grab the bank slot day of assuming you have the crowns. Unless you mean gold, then I see no real reason to wait.
I highly doubt there will be a bank upgrade in the rewards, but if there is that'd be nice. Part of the reason I was reluctant to get the discounted riding lessons. If I end up getting em in crown crates instead, then hooray.
Why buy with crowns something you can get with gold??? Is not it kinda ...counterproductive? Bought with gold means my character progression. Bought with crowns - just money spend on hobby.... But its personal decision, sure