I can't imagine the difference in airflow would be THAT significant to justify using two fan grills; and you could always try to make the grill-cross section (in the design to the right) even thinner. The reasons why I think it is better with the right one are:Furthermore I made two versions of the middle plate. I will test both on the prototype:
The left version is the new one the fan's will be covered by two 120mm fan guards/grill that will be included. The cover is necessary to prevent PSU wires falling in the fan. Also the left version will have a higher air flow through the middle plate. The disadvantage is, that I have to think about a new way to mount the bottom HDD bay, because the left fan guard will block the mount points.
The right version will have not the same high airflow like the left version this could be irrelevant,but it will provide a better stability for the case and support for the current HDD bay version.
If you take a deeper look on the picture you will see that I add some slot holes to the riser to increase the airflow in the motherboard area.
What do you think about it?
I stand corrected then.@K888D: You are right. I made a test. I cutout my hole design for a single 120mm fan on a cardboard with a plotter. With the cardboard attached the fan is much more noisy. Here is a website that comes to the same result:
So for noise the grill is the best option.