Free'er flowing - the alternative which is Crush bends have reduced diameter where the bends are because they are 'crushed'. A mandrel bend is a pre-formed bend created by putting a mandrel through the pipe so that it keeps the same diameter. More difficult to create tho, so costs more, takes more time to make the exhaust as well cause they dont just grab a big bit of pipe and bend, more welding.
Heres a few shots for comparison:
Crush bent (random off a search)
You can see how the diameter of the pipe is reduced around the bends.
Mandrel bends (my own)
See how the pipe is consistent, even around the bends. Make sure its painted or coated in some way if you get mild steel, if you dont it'll rust. You can see theres a mild steel flange on there thats been painted. Oh and yes I know its not a good idea to route it past the suspension there, but no probs so far, exhaust place did that, wasn't my idea.
Just so you know, it was $800ish to install that not including mufflers which I supplied, its also stainless and included tuned headers install, so for you should be like $300 for mild steel + cost of mufflers. Don't forget a mid muffler or 'resonator' whatever you do... or it'll be unlivable.