Nylon doesn't gouge as well as metal strands, I find, but it doesn't scratch the innards of your firearm either. So if you want to keep something pristine, nylon's the way to go. Just takes more work.
Steel is stronger and would likely last longer but bronze conforms better to the shape and direction of the barrel. So it's up to you. I'd say go with bronze, but steel I'm sure would work just as good.
Wipeout - from my experiance with custom and lapped barrels, does not clean the throat area well. There is an area of carbon that bulds up in the throat area that wipe out does not clean. I use a nylon brush and GM Carb cleaner.
Wipeout (great stuff, and now we can buy it in Canada rather then trying to get it mailed across the border, woo hoo!).