For loudness go with Magnafow, but for an awesome low rumbling idle that sets of car alarms on parking garages,
go with DynoMax!
I've got the stainless cat back system with the Super Turbos on my 78 SA and I love it. I get complimented all the time about the sound, and also experienced a very noticeable
increase in torque over stock.
The DynoMax system just f-ing ROCKS!
I would also recommend replacing the OEM cat with a light weight high flow unit (if you haven't done so already) for even more g's upon acceleration.