I live in Canada where it often gets below -20c in the winter. I have a cheap electric water heater (eco smart) and it does our house of two people fine. It’s a little slow to fill our tub, though. Cold weather has nothing to do with tankless vs storage water heaters. The company we hire to do our plumbing and HVAC installs in new homes we build often recommends Navien gas water heaters. They work great at high flow rates but require direct ventilation of the combustion gas. The last two homes I built used Navien tankless heaters and the home owners are extremely happy with them. Talk to a different installer in your area. HVAC contractors are probably to be the most knowledgeable.