These are two different types of radios. Understand that anything based on 802.11 is geared to shared access. 802.11 is TDD. Real microwave is point to point, FD, with varying types of modulation and encoding. There is no need for shared access, thus the design can facilitate high rate transmissions.
802.11a/g has a measured efficiency of .9 (bit/s)/Hz.
802.11 has a measured efficiency of up to 2.4 (bit/s)/Hz.
Common microwave systems typically achieve 8-10 (bit/s)/Hz.