the 690 has a fan mounts on the bottom two on the side one in the front,two on top thats airflow!
Do you really need the fan mount on the bottom (If you gonna do anything with it, its intake. And that will suck a lot of dust.
The 590 also has 2 on top, one rear, one front (on HDD bay, so its adjustable, as the fan on the bottom would hit the PSU with cold air, if you raise the HDD cage a little, you will get better airflow around the case) and it has 2 on the side.
Check this
Here is the CoolerMaster 690 HDD rack
Coolermaster CM 690
See how the fan is limited to cooling the HDD's only?
Where as the 590, the fan can blow air through INTO the case...
CoolerMaster Centurion 590
The rest is the same. Basically, from there on, its looks.
Also, one thing the 690 has, is the smaller holes ontop the top fans, so that dust wont fall in. But If you put top fans, thats not a problem...