I looked into that myself, and I guess the performance is pretty similar. MSD is THE name in performance ignition systems, just like ARP is THE name in fasteners, although there are other companies who make high performance fasteners. I know that the Mallory unit does not need the the seperate limiter chips like MSD does. The unit can be adjusted to cover a wide range.
That said, I went ahead and popped for the MSD 6AL. MSD has a great reputation for reliability, and are easy to deal with when there is a problem. I don't know if Mallory is the same.
Kris