John, the first thing that you have to do is check out what is required by the code in your area. Neither of the options that you mentioned would meet the code in our area,or in most of the country, for instance. You have to use black pipe in most of the country or you can use special flexible tubing that you have to be liscensed to install. Check with your local building inspector. An alternative is to go to a good local plumbing and heating specialty store and they will probably know also. The flexible tubing that you mention is usually only acceptable for appliances that must be moved like stoves and dryers.