Magnaflow VS Flowmaster?

[WuchaGot]

I am betting this is a Apple VS PC or Harley VS Yamaha type question that is highly opinion based but maybe I can get some solid tech spec advice and not just preference of sound (I am interested in opinions of sound as well.)

I want a nice low idle rumble and I want to hear the power when I take off but I don't need everyone in a 3 block radius turning their head when I come into the neighborhood. I want performance and/or fuel efficiency improvement, not just scull shattering sound. I know fuel economy and performance do not go hand in hand but looking for info on either or.

Opinions of true dual over single to dual?

1987 k20 single to dual currently and interested in going true dual.

Finally, I was kinda leaning to Magnaflow, what is the recommended Magnaflow muffler in a true dual set up? inlet outlet pipe size and such.

Thanks yall