Black pipe in wall is good. Make sure on the heater end in the direction of flow you have a shutoff valve, a union, a drip leg and then the connection to the unit. If using a flex connection, check the heater's directions as some equipment prohibits a flex product within the unit's interior. The wardflex and trac pipe is an option and can also be installed behind drywall. It's sale is restricted to licensed plumbers due to training issues so I am told. Although the 150 plus houses I built had wardflex and trac pipe, I hate the stuff. Although fast to install, I have had too many problems with it in production homes. Trim nails readily puncture it, excessive twist during fitting tightening damages the corrugations resulting in leaks, etc. I know those issues can be traced to poor install/technique, but I would stay away from it personnally, your mileage may vary.