A lot of people will tell you the following...
"If you have found airfare that you are happy with and would normally purchase, get it"
If a ding comes out you can buy those and have some credits left, granted they would have to be used within 12 months from the original purchase date, not your flight date or your change date. If you dont buy there isnt anyway of being sure you would get a ding, or that it would be cheaper. Prices could go up and then you would kick yourself for not buying when you had the chance.. If the 15% off is still a little more then you are willing to pay / want to pay, then play the waiting game and hope..