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.
"Don't let them promote you. Don't let them transfer you. Don't let them do anything that takes you off the bridge of that ship, because while you're there... you can make a difference." - James T. Kirk