Another advantage of copper/nickel tubing is that the material is more malleable than steel (and a lot more than stainless steel) making the flaring process easier. You are less likely to crack the flare and the steel nut will deform the flare easier making it less likely to leak. I have used this material for two complete truck restorations and half the tubing on another and there has never been leaks or failed flares.
Kent

1937 Chevy 1/2 ton
1942 Chevy 1/2 ton
1947 Diamond T Model 509
1951 Chevy 1/2 ton
1950 Chevy COE Model 5700 ~ "Barney" ~ And more pix