ATc-610 is full aluminum construction.
ATC-620 is steel frame, panel and aluminum front panel.
both cases are identical in construction.
ATC-610 do have mirror black paint finish for the black one.
Cooling wise, it has only one 60mm rear exhaust fan.
Both sides have vent holes for cold air to move in.
You can only screw down one side of 5.25" bay.
The other side is held by spring load tab or something liek that.
Overall, I am very impressed with the case. It is similar width as my receiver and I was able to fit the whole unit into my small stereo rack. It blends in well with the rest of the components.