Loaded question, so here is the answer.
In brief, What you pay is what you get(no offense).

I have Eisenmann Race exhaust on one of my BMWs and I have closely examined and heard Dinan and Akrapovic on few occasions. Akrapovic just stands out in comparison to others.

If you look at construction of three, Akrapovic is the most detailed and intricate, with better material(Titanium), workmanship and quality. Dinan is the cheapest looking of three.
Akrapovic is the lightest of three. Dinan for some reason does not state the weight of their exhaust, but is unofficially known to be heaviest.
Akrapovic is biggest setup(see pics below) and they also proivides the detailed information about their exhaust.

See pics and videos of 3 below.

You can get better than list prices on Eisenmann and Akrapovic.
Best comparison is seeing and hearing the three in person. I am getting Akrapovic next time.
BMW also uses Akrapovic on their Safety cars...see videos below.

Comparison chart:

----------------HP Gain----------Torque (Nm)---------------------------Weight (lbs)
Akrapovic ------11---------------11 lb-ft (15Nm) @ 2,270 rpm ----------50.7
Eisenmann-----14(Unofficial)--------?----------------------------------------53.2
Dinan--------------8 -----------------7 lb-ft @6000 rpm ------------------------?
Stock -----------------------------------------------------------------------------103



Dinan





This is X6 50i video...No Video of Dinan on X5M found.



Eisenamann








Welcome to Eisenmann North America
Eisenmann power gains - Eisenmann BMW E92 M3 Dyno Day


Akrapovic







http://www.akrapovic.com/typo3conf/e.../m_bm_t_1h.pdf

Akrapovic on BMW Safety cars

1M coupe Safety car


F10 M5 Safety Car