The Heat Pump Water Heaters are pretty good in the energy savings, pretty much a no-brainer as far as them paying for themselves in energy consumption. I'll get into that later, first I'd like to address the smell.
I'm guessing the smell is either a musty smell or, a rotten egg smell you didn't give specifics however they are the common ones. These smells are caused by water conditions with either high iron (musty) or, high sulfur (rotten eggs) resulting in growth of either a iron reducing or, sulfur reducing bacteria that causes the associated odors. There are remedies that include water treatment and anode replacement in the water heater that will cure the odor problems. Water heater replacement alone will not do it so I want you to know up front that the odor is not a reason to replace the water heater.
Now as far as the water heater...
Water heater brands: I would not blindly say that AO Smith is the best out there. AO Smith is a large company that includes other brands such as American Water Heaters, State, Reliance, GSW, John Wood, & Apollo as well. Some models are good and some models are the biggest lemons ever forced down the throat of the unsuspecting buyer as well. They make the
infamous Whirlpool Flame lock and Flame Guard water heaters
sold at Lowe's which resulted in a class action lawsuit and they also make the Energy Smart Electric water heater as well. As for the Heat Pump water heaters they all have a 10 year warranty and it is too early yet to say which is the most reliable or, if any unit has problems.
The actual payback you receive is dictated largely upon where you live and the ambient temperatures you have. In the extreme south the units can be used in heat pump mode almost 90 - 100% of the year. Midway up the country 70 -90% of the year and in the northern states about 60%. The unit needs at least 40 degree f air to operate and lends a cooling effect to the area it is located in which must be at least 1,000 cu ft in volume.
If you are in the warmer areas of the country I would consider the Stiebel Eltron Accelera 300 Heat Pump Water Heater
which relies heaviest on the heat pump to produce hot water and draws only 9.8 amps with a 1750 watt resistance element for backup for the highest savings.
Midway up in the country the AO Smith Voltex
would probably be the best with its 4500 watt upper and 2000 watt lower elements as back up for the heat pump. This offers a mid range savings as far as the heat pump water heaters go.
In the northern part of the US the GE GeoSpring, and Rheem/Rudd water heaters which have 4500 watt upper and lower elements offer the best possible back up for the 40% of the year when the heat pump cannot operate. Remember if you have these units in a heated area you are making heat for the heat pump to use.
I hope this helps clear things up for you and helps your decision process.