Between MicroFly and Q-Pack, I'd go for the MF - that extra 1" means a lot. Between the MF and the Aria, they're two different animals. I had considered both, but after hearing about the clearance issues with the Aria, I went with the MicroFly instead. This was mainly since the MicroFly was to be my main computer (temporarily), and I couldn't see myself shoehorning an aftermarket HSF and high-powered vid card into an Aria. Looks like the MicroFly was geared toward the hobbyist - it's light and able to house a bunch of stuff.
That said, if you're looking for a quiet computer, the Aria's probably a better bet. If it's for secondary use, beside a TV, it'd be a much better choice than the MicroFly.
One more thing: I second the fact that the MicroFly's side panels are perfect for transmitting sound to the outside. Additionally, the side windows, if equipped, don't have vent holes, making the interior a bit warmer than it should be. You can get the panels without windows, and in that case, the side panels get vent holes.
PM