The best answer I can give as to whether or not there is a difference between mufflered or muffler-less..... :--) Oh God Yes!!!!
From inside the cab, I really didn't notice any difference. But, if you stand back by the tail pipe, even at idle it is MUCH different sounding. And if you drive past a wall or something that will reflect the sound back to you, roll the window down (passenger side window) and give the go pedal a little extra tap, and you will be



There is a deeper rumble sound at idle w/o a muffler, much more like a big rig sounds. Under acceleration, the sound from outside the cab is just wild! It is a very, very throaty rumble/scream type sound. Think of the meanest sounding V8 gasser in a car, like a Mustang or Camaro. Now multiple that by 5 times. You get the idea.
Some people don't like it, I will never go back to a muffler if at all possible to stay straight piped.
I have the 4" Pinnacle Performance from Jerry Jardine exhaust on for about a year, then removed the muffler last April, so I have a good long time with and without a muffler on mine.
The only cautionary warning I will give about mufflerless - I personally have a very annoying drone in the cab from 2000 - 2300 RPM without the muffler. Some folks get it, some don't...
Since you just asked about which gauge metal is better - I personally say the heavier the better. A heavier gauge system will of course most likely last longer, but it will also help reduce, or even eliminate the drone you *might* get if you go mufflerless.
Tom