How old and how efficient is the boiler? Older ones tend to be inefficient, but that is not always the case. Tankless size depends on peak needs and worst case incoming water temp. The more you need at once and the colder the incoming water, the bigger the thing needs to be (and that may require redoing your gas service - don't even think about trying this with electric). You might have to upgrade the gas service all the way back to the main line, and that could be expensive. Maintenance on a typical tank is close to zero (although you should probably drain the sediment out of it periodically, not that many people do). If you don't do regular maintenace on a tankless, you'll likely notice a degradation in performance fairly quickly. The newer boilers are designed to not maintain some minimum temp, so they typically only fire when there's a need. Older ones can't handle that. An indirect would approach your boiler's efficiency during the heating season, but if it can't cool off during the summer, the standby losses mount up. If you could throw drainwater heat recovery into the mix, tankless looks better, when used with a shower where the heated water is constantly going down the drain. This will help keep the size required down some.