In many (most?) cases, the tankless heaters require both a larger gas/propane supply pipe and a larger flue to vent the exhaust. This was the main reason I decided to stick with the tank style. There have been articles in Fine Homebuilding and Journal of Light Construction over the last several years. One article by a reputable plumber was an in depth discussion about how he installs these systems with an auxillary tank. I couldn't stop laughing long enough to actually read the article to find out if it was serious.
The problem with education in the School of Hard Knocks is that by the time you're educated, you're too old to do anything.