It is difficult to give minimum wind strengths. It depends so much on what the wind is like, what the water is like, what the current is like, what your skill is, where you measure the wind, etc...
The big difference between the two boards is in the way that they edge. To go upwind well you want a long edge to give you a nice steady upwind carve. In high winds you want a small edge so that they can scrub speed. The AX150 gives a really smooth, efficient ride. The Ex138 gives a more loose ride.
One really interesting experiment we tried was riding the two boards being a boat. The AX150 was really smooth and steady. Huge carves and a really stable feeling. The Ex138 easy to spin out and break the edge out, maybe more fun, but not as relaxing.
In light winds you want the AX150 characteristics, in high winds the EX138 characteristics.
I think the actual lower limit in wind speed will be very similar between the two boards. But on the EX138 you will be fighting and on the AX150 you will be relaxing.