I currently have a very inefficient water heating system that I m attempting to upgrade. I have two showers, dishwasher, and a washing machine. In lieu of any existing tank I get my hot water from the coil of my oil boiler (hot water). I have talked to various plumbers and half suggest and indirect water tank that is fed off of my boiler (with the possibility of hooking the heater into an existing gas line to be utilized in the summer). The other half are suggesting a tankless water heater. What is the most efficient system.