I can't think of a con for MSD. Well, if you want to considering connecting 4 wires a con...
I have the MSD 6AL and love it. Very good investment. If you have a 5 speed, get the 6AL because it has a rev limiter on it. If you have an auto, you'll be fine with the 6A. They are the same box except for the rev limiter. MSD is great because everything plugs right in with each other. I have a retard as well, and it pluged right up

Performance advantages: you get better MPG, quicker throttle responce, more HP, and easier starting.