I don't have either but I'm going through the same process, and also considering m perf and borla atak. A big benefit of the akra is having an upgrade path to full Evo down the road.
I have only 3500 miles on my car and don't want to risk my warranty yet with the full system.
Another factor in my mind is that even though a loud exhaust sounds good at first, it can get old with time. I traded a mustang for the bimmer to have a car more balanced and refined and I don't want to try to make the m3 sound like a muscle car which it is not.
Also I'm afraid the drone of the eisenmann race will get to me. I had exhausts with drone on older cars and it made me literally sick, I know some people react to it differently than others.
Anyway, just wanted to share some thoughts from a guy going through the same excruciating decisions.
2015 M3 6MT | Yas Marina Blue, CF Roof, Cloth | Adaptive M Susp, 437M 19" Silver | Stripper
2013 X5 xDrive50i | Carbon Black | M Sport + M Perf, 333M | Loaded