I have the Akra Evo on my M3. I have no experience at all with the Gruppe M exhaust. Based on this, I would say the Akra is undoubtedly the better system.
2011 Le Mans Blue M3// Black extended Novillo w/ Blue Grey Aluminum trim/DCT/ZTP/ZCW/ZPP/ZCP Premium Sound ///Akrapovic Evolution Exhaust
2012 Le Mans Blue 135i M Pkg DCT
2008 Brilliant Red Audi TT 3.2 Quattro 6M