I love stainless... leave it bare and it won't rust! And welds will stay colourfull. I used Solar Flux type B to shield the inside of the weld.
Mild steel, and it's rusty already! The last section of the exhaust (the pipes over the diff to the rear of the car) is mild steel. Stainless can sound a bit 'tinny' apparently, and I want a more mellow/deep sound. I save a bit of money too. These will be sandblasted and painted in high temp black paint, then the bends will be wrapped with thermal wrap where they pass by the shocks and gas tank (mainly 'cause I have some unused wrap, and because it'll look cool).