In my case, side exit is because there is simply NO room to go over the axle at all. It separates the sound pulses and results in a less-pleasant sound, in my opinion.
My other "fast" (not that fast) car had hooker aeros dumped in front of the axle, but they were along side of the driveshaft, and had turndowns pointed towards each other at about a 30 degree angle. Much more pleasant sound, imo.