AQ6 still has the advantage of having multiple motor/pwm outputs right on the board. With M4 you can get 4 PWM outputs but you need to tap into the expander pads (J1/J2), so anything larger than a quad would require CAN ESCs or an adapter like the AQ "motherboard." Or if you want a brushless quad with gimbal/passthrough outputs. AQ6 also has SD card onboard (M4 requires an expander board for that), and 2 serial ports (vs. USB and one serial). AQ6 has a much larger input voltage range. AQ6 with DIMU is incredibly stable, and now very simple to calibrate as well.
The M4 is an unbeatable value though, and I enjoy my micro quad tremendously! It flies much like a full-sized one yet can take off from my desk (remember to unplug USB first!
) and land in my hand w/out danger of losing fingers. With a simple .4g home-made dipole GPS antenna I get full autonomous capabilities as well. At that size/weight crashes are no big deal, and the frame/motors cost peanuts. It's fun, and a great development/testing and flight practice tool. The fact that it could also be used on any size MR is of course a major bonus, but if you're building something larger in the first place, the AQ6 might be more practical in some ways.
That's my current personal take on it, anyway.
-Max