There are two recent posts that are very similar. I'm sure this question comes up often and I figured I'd respond to both for archive/search purposes. The following is a comparison based on the manufacturer's stats/websites, as well as a synopsis on my exhaust setup.
Gotta love Magnaflow's straight through design, materials, and low cost. Full stainless housing, core, deadening mesh, AND welds all for a whopping hundred bucks...
http://www.magnaflow.com/wideopen/wideopen.asp
Dynomax is not bad. BUT... they don't have as much packing material - no stainless mesh. It is unclear whether their perforated core is stainless.
http://www.dynomax.com/products/ultrafloss.html
No offense to anyone with Flowmasters but baffled/chambered mufflers are a complete waste of time. The design is strictly for sound, (click above dynomax link - shows exhaust flow comparisons ). If anything, the design increases turbulence and backpressure. Anyone with a science background will laugh his/her a$$ off at the following BS:
http://www.flowmastermufflers.com/flow_technologies.html
http://www.flowmastermufflers.com/images/deltatech.jpg
The above picture is ridiculous...
Cherry Bomb's Vortex is also a chambered exhaust, but has a better design than Flowmaster. It comes with aluminized mild steel... which in terms of longevity, is a waste of time.
http://www.cherrybomb.com/image.aspx?image=vortex-cutaway_lg.jpg
SOUND CLIPS
http://pnw4runners.com/exhaust/exhaust.htm
I have the 5x11x28" Magnaflow in polished stainless steel with mandrel bent 2.5" stainless pipes. If I'm not mistaken, the polished version comes in 304 stainless unlike the 409 stainless on the satin.
It is just a little louder than stock. Slight rumble at idle. No annoying drone at 2000 rpm cruising. Roars at WOT.
I HIGHLY recommend you go with Magnaflow from a materials/cost/performance standpoint.