Beyond the basic offerings, I've found the cost of more efficient units is incommensurate with a reasonable payback. Compared to home heating, a domestic hot water heater is used little. Your smallest summer gas utility bill typically represents hot water use and cooking. You only get return on efficiency if you use the appliance. The less you use it, the less return, but the life of a water heater is based on years not use.
The 40 gallon hot water heater provides the best first cost. The better units have longer guarantees and commensurately more cost. Unless your water is severe, I suspect the basic unit with six year guarantee will easily last 10 years, but I'm not sure one with a 12 year guarantee will last twenty years.
Gas 40 gallon 6 year runs about $400
Gas 40 gallon 12 year runs about $650
Gas 50 gallon 10 year runs about $550
Gas 50 gallon 12 year runs about $650
Gas 50 gallon 6 year direct vent runs about $1200 - I suspect greater efficiency
I see a POWERFLEX 50-Gallon 6-Year Short Gas Water Heater (Natural Gas) for $900. This appears the most efficient and uses PVC vent.
A larger heater provides more storage and greater recovery. We replaced a 50 heater with a 75 gallon unit. The 50 gallon was a bit lean for our soaking tub and two teen showers meant the third waited. I've never noticed a shortage with our 75 gallon and we even use it to heat our 900 sq. ft. basement.
Flue size and connected capacity must be checked against the gas code if you are upgrading to a larger unit.
I've seen 50 gallon high temperature units that use a heater mounted tempering/mixing valve. This provides more capacity, but again requires flue considerations and costs are much greater.
Instant hot heaters have the comfort advantage and cost disadvantage of never running out, but at a grand and additional installation costs, they are difficult to justify with USA utility costs. Energy savings come from lack of losses from a standing tank. IMHO that's overrated. Put your hand on the tank and tell me how much heat your loosing. I'm also not sure if you can get commensurate flow (gpm).
I also see standing pilot's becoming out of style. Ours has a Piezoelectric ignition does not require external power or battery. How it works I don't know.
I'm assuming you have gas. Electric units are 100% efficient (less heat loss) and less costly, but will quickly cost more than gas due to utility costs. Electric is less restrictive with respect to location because no flue is needed. Our vacation home has an electric 40 gallon unit packaged to look similar to an appliance and sits next to the washing machine. This enabled us to remove the enclosure around the old one and pick up some space in the laundry room.
I agree with replace before failure. None last forever and when they fail you still have replacement cost on top of the collateral damage which may add several times the cost.
With respect to the above article, the following is a better read: http://www.hotwater.com/naeca/