140:
Has mounts for two drives.
Cross flow fan, underneath exhaust.
Dedicated USB1.1 controller
240:
Has mount for one drive. Additional drive requires bracket kit.
Has rear exhaust fan
Dedicated USB2.0 controller.
FWIW 540:
Hardware set up like 240, except smaller board, and USB 2.0 port controller and video decoder is built into the CPU chip.
The 240 is preferred becasue of the dedicated USB2.0 controller.