I personally prefer Magnaflow over Flowmasters...I've had both and while the quality is great on each, I simply prefer the low rumble of the Magnaflow over the loud, raspy Flowmaster tone.   Of course, different cars and different engines can change the sound.  Whatever sound you prefer is what you prefer.

If you want a less expensive, non-name brand muffler, I would suggest getting something like Summit Racing's house brand.  At least you know who is standing behind them.