We don't have much info to go on. We don't know what your needs are, what your wants are or what your budget is. All we really know is that you want gas in the event that the electricity is out.

A tankless requires a small amount of electricity to operate - it would not operate if the power was out. A generator would solve that problem and provide power for other things as well.

I don't completely agree with Dan, you can size a tankless to run more than one thing at a time. I sized mine so that in the coldest part of the year I could run the shower and the bathroom faucet - in the summer I can run about twice that amount. It works fine in my house, but if I had a larger family I'd go with a larger unit.

There are also 50 gal Hybrid units - They store the water but have high BTU input and very high efficiency. They can produce a certain amount of continuous hot water as well.
These meet the 30% tax credit requirements and vent with PVC pipe - making them cheap to install where long vent runs are needed.

I have included an example - I'm not endorsing a particular brand.

Note that these would not run without a small amount of electrical power.

Here is a link that might be useful: State Hybrid

Thanks for the input. I've been reading up on tankless and I think we'll pass on that option. Thanks for the info on Hybrid units...off to check 'em out.

Are you restricting yourself to one manufacturer? We have had a Bradford-White 75 gal (huge Jacuzzi) for almost 15 years with no problems at all. I think they are manufactured in PA. We have a power vent so in the case of power outage, we don't have hot water either despite having a gas heater. The stored hot water in a 75 gal tank was enough for 4 showers the next morning so that usually is enough to get us through.

Just checked in with the builder again. I misunderstood the problem. It's not the actual water heater availability that's the problem, it's the power vent that goes with it. And now I learn that with this type of venting, we'll still need electricity for the gas water heater to work. CRAP! That's the whole reason I wanted a gas water heater.

Thanks for the info Susanelewis.

You need a power vent when you cannot exhaust the fumes to a nearby chimney. That is the only circumstance that I'm aware of where you don't rely on electricity for a gas water heater.

you can get tankless heaters that allow more than one major appliance to run. They are "Enough to run 2-3 major applications at once, direct vent room-sealed combustion, built-in power vent allows for vertical horizontal vent termination"

I still dont understand what is there to think about when you have to choose between tankless and tank - 70% of the time you are not home or sleeping or not using any hot water. You safe power all that time. True, it will not work when the power goes out. But - when the power goes out, I am not taking a shower or trying to do dishes. At a max I will wash my hands and I can do that with cold water. But I never have to "plan" hot water usage. We had 6 adults in our house for two weeks, eating, cooking, washing dishes, showering, - not a single second of worrying.