I went with the mild steel... IMO the mild steel handles vibrations better and the SS have a tendency to crack. Also, since EVERY set I've heard of needs a little tweeking to fit right, the mild steel is easier to bend that little bit if you need it.
not cracking... would never seat good and seal, got ss on my 12v and no troubles... a rigid tubing bender is the only way i found easy to bend ss, but if u take your time and care alil you can make ss. work, mine aren't perfect but i got in a hurry cuz truck was down longer than expected