When using threaded pipe you rely on tightening the pipe threads to seal the joint. If you use the PTFE paste (joint compound) when assembling the black iron then you most likely won't have a problem with leaking. The teflon tape gets shredded by black iron threads so it doesn't tend to work as well. Shredded tape equals increased friction.
If you use black iron, install a union fitting when coming back down the wall so that if you do have a leak, you can just undo it at the union and work from there instead of taking everything downstream back apart.
Another positive with black iron is that the increased mass in the pipe acts like a condenser / heat sink and stays cooler longer. Use at least 3/4 thoough as this will keep pressure/volume drop low over distance.