If you are already running an older low or mains pressure cylinder, then it often makes sense to replace with another cylinder. I guess it all depends on the size of your family, your budget or if trying a more sustainable option is something you want to consider.
A heat pump water heater is actually another solar device. It just makes use of free energy in the air (thermal solar) rather than relying on sunshine (solar UV). It does this night and day, even if it’s raining and -15 degrees air temperature outside.