You didn't say, so I'll ask:
Do you have a hydronic (forced hot water) heating system?
If not, ignore the following.
If yes...
The best HW heater you can buy is an indirect-fired HW tank running off the boiler.
And if it's a so called "reverse-indirect" (eg. ThermoMax, ErgoMax, Everhot EA-models, etc.), plumbed as a buffer for the heating system rather than as a separate "priority zone", so much the better. Buffering the heating system will yield heating season fuel savings in double-digit percentages, and in the off-season overall efficiency will be no worse than a typical gas-fired tank. If plumbed & controlled as a separate zone it'll still save some fuel during the heating season, but not as much as in the fully-buffered heating system scenario. (Exception: If you have a very low temperature radiant floor-slab heating and a modulating-condensing boiler the buffer temps would be higher than necessary for the slab, and the overall efficiency could in some instances be a few percent lower unless the HW tank is zoned separately. If the heating system needs 130F or above most of the time, buffer it with a reverse-indirect HW tank.)
It'll be 1.5-2x as expensive up front, but it'll be the last hot water heater you'll ever buy- it'll outlast the boiler and then some. Even smaller ones can deliver as much heat as a tankless HW heater as long as you have a big enough boiler behind it. The size of the tank is less of an issue than the size of the internal heat exchanger- a 25-35 gallon indirect will usually provide more first-hour gallons of 120F water than a typical 50 gallon self-standing tank. (The exact first-hour amount or highest continuous flow rate is determined by the boiler output, storage temperature, and heat-exchanger size.)
FYI, on net efficiency:
http://www.nora-oilheat.org/site20/uploads/FullReportBrookhavenEfficiencyTest.pdf
Reducing the number of burners (1 instead of 2) reduces the overall maintenance, and the size of the required combustion-air capacity, reduces backdrafting hazards etc. etc. It's really the "right" thing to do on so many levels whenever there's a hydronic boiler driving the heating system.