or is it asthetic. I have a 6" duct in my basement ceiling that falls between a joise and suspended ceiling grid making options for droping it thru a 2 x2 ceiling tile few....

narrowed it down to two options...

1) have the 6" duct drop down over backside of 2 x 2 ceiling tile into 6" round diffuser. Diffuser thru tile. Th e6" grill thru front of tile into diffuser. So I end up with a 6" round grill.

2) By a boot that id 10 X 6 but with the 6" opening for the duct facing the short (6") side. This will fit my space. I'd have to fashion 2 hangers out of 2 x 2s and attach the boot to the 2x2s suspended on the grid system. The 2 x 2s would also offer something to screw th 10 x 6 grill into.

Atshestics aside...what is the difference in air flow / comfort between a 6" round and 10 x 6 grill?