Re: Where to spend my gems?
Original plan! 🙂
To get the most use out of your cards, even those fancy new Galactic Gardens cards, you'll want to be able to utilize any and all available combinations, meaning that you'll want every plant and zombie hero unlocked. There are also hero quests to complete, and that'll give you a few gems and sparks. 750 gems for a guaranteed hero is an absolute best-purchase in this game*.
*I'd add an "IMO" or "IMHO" disclaimer, but it might be a fact.