Resistance heating is so 20th century. Granted, you likely cannot do a DIY air source heat pump build, but the COP is so high for such systems, that it's probably worth it to just buy it.
Hopefully the new heat pump water heaters are better. The advantage of resistance heating is simplicity and cost, with no moving parts. Solar panels are so cheap now they make it hard to justify the expense of the heat pump, assuming you have room to mount the panels.
it disappoints me (but thrills me) that improvements in PV efficiency and cost have made solar thermal hot water more or less pointless.
Losses are higher but you store more energy per L, which is often the limiting factor.
I think that what actually costs money is not the space but the tank. Higher temperatures mean not only more expensive materials and shorter lifetimes for tanks and piping but also higher conductive losses.
1. Consider PCM heat storage (still relatively new technology, but works well with heat pumps)
2. Maybe the problem shall be solved at the building level, not individual apartments.