Hmm, doing the math on spending 20 bucks:

Currently keys are 15.02% off: 2000/956 *10 = 20.92 Master Keys.

If you got 15% bonus Zen after the sale: 2300/1125 *10 = 20.44 Master Keys
If there was a 15% discount on Zen, you'd essentially be getting 17.65% bonus Zen: 2352.9/1125 *10 = 20.92

If it's a question of cheaper keys or cheaper Zen, you get more bang for your buck taking advantage of the sale.

Of course, ideally you do both:
15% bonus Zen with the sale: 2300/956*10 = 24.06 Master Keys
15% discount on Zen with the sale: 2352.9/956*10 = 24.61

If we wait for both the discounts and the sales, we get 20% more keys. Excellent, I've just talked myself out of spending money.