I'm going to go ahead and try to protect you from the frenzy that may ensue at any moment. If you search for akrapovic and MPE you will find pages upon pages of comments on this site.
I will sum up what they will tell you:
hp difference is negligible.
sound is subjective.
since you're considering the akra slip on the real issue here is sound, not performance so you need to decide for yourself.
Some may say that the MPE is covered by warranty but I don't see how that's an issue for a simple axle back system. Both are high end, it's not like it's going to fall off when you hit a pothole. Moreover, if you find the right dealer they'll install the akrapovic system for you.
Oh, and I don't own either but I spent about 3 hours yesterday reading the threads to try to answer this exact question.
End result for me? I still have no idea. But, my SA at BMW has the Akra Evo system and the ESS tune and once I have the chance to hear it I'll make my decision.