At Portland's gas & electricity rates the operating costs of a gas-fired hot water heater would be dramatically less than that of an electric tank. Since you mentioned gas, I'm going to assume you already have natural gas service to the house, which makes a HUGE difference.
Like a standard electric tank, or even the Marathon, an electric tankless is more expensive to run than even a legal-minimum-efficiency gas fired tank in your location, and has serious gallons-per-minute issues.
Heat pump water heaters will only be dramatically cheaper to operate than an electric water heater if your space heating system is gas-fired, but if your heating with electric baseboards or similar the annualized savings are pretty miniscule. These units take half of their heat out of their surroundings, and in a heating dominated climate that heat needs to be re-supplied to the space with the heating system. There are reliability & noise issues with heat pum water heaters too, being much more complicated beasts with more moving parts than any tank heater. In general heat pump water heaters make the most sense in cooling dominated climates, since they actively cool the space where they are located. In Portland OR that space-cooling effect only has value 80, maybe 100 days out of 365, and for more than 200 days you need to run the heating system to make up the heat deficit created by the HW heater.
A Marathon or HP heater would only make economic sense if you didn't have natural gas available. The difference in operating cost would pay for a new gas-fired heater in well under 10 years, and even bargain-basement gas heaters tend to last longer than 10 years.
Don't even think about it- if you have natural gas service and the lifecycle operating costs matter, move on- gas is just too cheap relative to electricity!
A gas-fired tankless only makes sense if you have a monster tub to fill, like a spa or hot-tub, or if you desperately wanted to recover the space that a tank takes up. These beasts are as complex as modulating-condensing boilers, and have their own special set of maintenance issues to attend to. The water-hardness in Portland is low, and you'd likely get good life out of them, but it's unlikely that you'd recover the upfront cost delta between that and standard tank on fuel savings over the lifespan of the unit (unless gas prices triple.)
If you want the high efficiency of a condensing tankless for a lower installed cost, a condensing tank like the Vertex has comparable or better performance and less installation & maintenance. Like a condensing tankless they are direct-vent, meaning they take their combustion air from the outdoors rather than pulling in air from the outdoors. Standard atmospheric-drafted tanks use conditioned space air for combustion, and have an open flue de-pressurizing the house 24/365 increasing the heating & cooling loads. (That's probably less than a 2% bump on your heating bill in Portland though.)
If you're heating this place with a gas-fired hydronic or steam heating system you make get some synergy & lower fuel use out of installing an indirect-fired tank for the hot water running as a heating-system zone. The effective efficiency this buys you depends on the particulars of your boiler, and it's output relative to the space-heating load. This is usually a better overall bang/buck on efficiency& longevity than standalone tanks or tankless units of any efficiency rating. If you have a boiler rather than a hot air furnace and it's a lot newer than a 1926 coal-converted-to-oil-converted-to-gas beastie running at only 50% efficiency, let's have the particulars to see if there's a good/better/best approach to take.
. There is so much information that my head is spinning – heat pump/gas/ electric, tank/tankless!?! I would like to be an educated consumer as this is a major appliance that I don’t want to have to replace for a long time...Marathon says they have a lifetime warranty! Anyone out there have any advice or good/bad experiences that they can share or other resources that I could use?
