I did not bother reading all of the replies but here is my .02. It depends on what your goal with the car is. If you want it to be as light as possible and able to certify quicker than 7.50 then there is only one choice and that is Moly. If you never plan on going faster than 7.50, and do not care about a few extra pounds but want a car that you can build and never worry about fatigue then mild steel is the way to go, and much cheaper to boot. However if resale is something you want to consider moly cars generally fetch more money for whatever reason. Although my last mild car sold for a good deal of $$$$ and I had a hard time giving away my last moly car