Magnaflow or Flowmaster, which one has the nicer sound and produces the most hp/tq on a tpi'ed 305 with minor mods, check my sig below.

 

I really dont think you'll see a noticeable difference in hp/tq between those two on a 305, but my choice between the two would be flowmaster for sound reasons. If you are concerned about flow, stay away from the crossflow designs and go with a single 3" out muffler. Those sound better yet IMO. I have a single in/out hooker aerochamber on my car and it sounds great

 

i put flowmaster catback on my car. the rest of the exhaust is factory and to be honest, the clutch fan was louder than the damn exhaust before and after the flowmaster... so i bought a cutout and solved the flow and the sound problem!!

btw- my flowmaster is single in, dual out at back bumper... if u get dual outs, get straight out pipes, i chose to go w/ the turndowns like factory exhaust and a friend of mine w/ an '88 2.8L got the same exhaust but straight out and his sounded so much better than mine!! although... mine looked better cause u couldnt see where the chrome tips welded to the stainless pipes

 

any "performance" cat back on teh market will add the same amount of power from the same sized tubing. buy whichever muffler brand you prefer the sound of.
personally i htink magnaflow stuff sounds really nice. it is what i plan to use on my car when i fab my exhuast for the new motor.

later
tim