Instant hot heater - give you hot water instantly after you switch on the power (ok, have to wait a minute of so) and turn on the shower. Some new water heaters comes with built in pump.

Storage water heater - will need to wait for the water inside the tank to get heated. The duration will depends on the wattage of the heater. This heater will give higher temperature compered to instant water heaters. To get good water pressure, will need to install a booster pump.