The ASU-II's are compatible with the MXe's (gen 1, 2, 3 and L) as well as the CX and CX-II platforms.
If you're replacing the MXe-III with an EX, you will need another solution for Analog devices. I use Grandstream GXW-4224's - they work very well. Mitel is late to the game with their similar product, and since we have 50+ GXW-4224's out there, we stick with them... but if you're doing this from start, then you might consider going with the Mitel platform.