The tank vs. tankless decision comes down to a genuine engineering tradeoff — it's not simply that one is newer or better than the other.
Tank vs. tankless
Tank water heaters keep a large volume of water (commonly 30–80 gallons) heated and ready at all times. Simple, well understood, and cheaper to buy and install, but they lose some heat continuously just keeping that stored water warm — called standby heat loss — even when no one's using hot water, and you can run out of hot water if demand exceeds what's stored (a full bath after several showers, for example).
Tankless water heaters heat water directly as it flows through, with no storage tank and no standby loss, and generally have a longer service life. The tradeoff: they have a maximum flow rate they can heat effectively at once, so running multiple hot water demands simultaneously (a shower and a washing machine at the same time, for instance) can exceed a single unit's capacity — sized correctly for the household's peak simultaneous demand, this isn't an issue, but an undersized tankless unit will genuinely struggle under heavy simultaneous use in a way an appropriately sized tank system wouldn't.
Sizing is about peak demand, not average use
For a tank system, sizing is mostly about total capacity matching household size and typical usage patterns. For a tankless system, sizing is about the flow rate (measured in gallons per minute) the unit can heat at once, matched against the realistic peak number of simultaneous hot water uses in the household — this is the number that actually determines whether a tankless system feels adequate or frustratingly limited, more than the brand or overall efficiency rating.
The upfront cost gap is real, but so is the operating cost difference
Tankless systems generally cost more to purchase and install, partly due to the higher-capacity gas line or electrical circuit some models need. Over the system's lifetime, the elimination of standby loss can offset some of that gap through lower energy bills, but the payback timeline depends heavily on local energy costs and actual household hot water usage patterns — it isn't a given that a tankless system pays for itself, and it's worth running the actual math for your situation rather than assuming.
The one thing people forget
Check the water heater's actual physical space and venting requirements before assuming a straightforward swap — especially for a tankless upgrade, which often needs different venting than an old tank system had, and for either type, a licensed installer needs to confirm the gas line, electrical circuit, or venting can actually support the new unit's requirements.