With one notable exception*, most indirect-fired water heaters utilize one of two styles of internal heat-exchanger -- either a copper finned-tube, or a smooth steel coil.
Avoid finned-tube heat-exchangers, especially if you have relatively hard water. The problem is that minerals within the water cling easily to finned-tubes, to the point that they become coated entirely in mineral-scale. When this happens, the mineral-scale inhibits the heat-exchanger's ability to transfer heat to the stored water, causing the attached boiler to short-cycle and operate extremely inefficiently.
Think this is just theory? Think again. Below is a photograph of an eight year-old BoilerMate with its finned-tube heat-exchanger removed. The white layers of mineral scale are clearly apparent.
We discovered this gem at a client's home. While he new something was awry with his heating system, evidenced by a slow, gradual climb in heating costs, he didn't think much of the fact that his boiler would operate nearly constantly -- turning on and off, on and off, throughout the day, as the water heater struggled to make hot water.
Once replaced by a Viessmann Vitocell 100 (the original H.B. Smith boiler was also replaced with a Viessmann Vitorond 100), the improvement was immediately noticed. The new boiler/tank combo runs like a top, with a huge increase in efficiency and an abundance of hot water.
Moral of the story -- avoid finned-tube heat-exchangers. Even if you're on a tight budget, there are plenty of low-cost indirect-fired water heaters that utilize a smooth-walled steel coil.
* The notable exception is Triangle Tube's Phase III water heater, which utilizes a "tank-in-tank" design. The Phase III is a quality indirect-fired water heater -- a good choice among stainless steel tanks.