I have built both type cars so here's my
It all boils down to what the car is going to be used for
If it's a slower type car not looking for 8's or better and never will, then mild steal will pass tech.
But the books I have from SFI (you can order them direct for 30 bux) says that after so fast you MUST use chrome moly
As for the tig welding mild steal, thats so the builder can charge more as stated above.