The most likely re-engining path for either aircraft would involve turbines with larger fans. The A320 can accommodate such an engine change while the 737 is already at its limits regarding fan diameter. This fact alone will drive Boeing's decision towards a clean sheet design.
Airbus will be limited by investment capital and engineering resources in the coming decade. They are no doubt cognizant of the competition's highly successful NG program derived from a then-30 year old design. That age milestone is fast approaching for their own A320 and a more efficient engine may be the key to the program's continued success.
Competition from Embraer and Bombardier on the lower end will no doubt be a critical motivating factor for both Airbus and Boeing to get moving forward soon.