It depends on your situation in the game also.
In the beginning, you can get a lot out of playing shop stories for 100 gems. Great equipment, much better that in the main shop.
Check it out here
Personally I started upgrading energy saver and experience booster only when I had really enough gems, and I did it from time to time, when I had enough of them.
But then, I don’t buy gems. For those who do, it’s a different story.