One is lightest and the other is heaviest aftermarket exhaust available for M3.
Both sound good in very different way.
If you're looking for performance (by saving weight as slip-on exhaust wouldn't do much in HP gain dept), Akra is way to go in my opinion. And the used resale value in the forum, akra is def. highest as well.
PAST: 11 E70 X5M | 08 E90 M3 | 08 E92 335i | 03.5 E46 M3 | 99 E36 M3