Instant geysers do not have storage tank. (they have only .5lt to 1lt storage tank). The rate of output of hot water is slow. If you increase the rate, water dosent have much time to stay in the tank and get heated up. I have both instant and 15lt storage geysers in my house. Instant one (rated at 2kW) takes around 5-6 mins to fill a bucket of water. The water is hot enough to take bath immediately. If I want really hot water (for washing clothes), I need to reduce the flow considerably and time to fill one bucket increases to 10 mins. The storage geyser (15lt, rated 2kW) dosent have these issues. It has a thermostat and I can set the water temperature. For regular baths, it again takes 5-7 mins to heat the water, to allow me to take shower directly. For clothes washing, each bucket fill takes 10 mins. ie Allow the geyser to heat for 10 mins and when it cuts off take the water out.
Efficiency wise, both are same. Though, in storage type, you will tend to waste more electricity if you are not consuming the whole lot of water that you have heated. Instant type will not give you enough pressure to take shower bath.
Gas geysers, though have their own disadvantages are capable of giving you instant supply of really hot water, hence allowing you to take shower bath as well. For a family of two, staying in hyd, I dont think you will be consuming more than 1 cylinder per yr. However, make sure that the bathroom where you are putting the gas geyser is well ventilated. Or else, put the geyser outside the bathroom, near the gas cylinder and run the water pipes for the distance.
Comparing geyser vs Immersion rod, (a) If you forget to switch off the geyser, it has auto cut off and you are safe. If you forget to switch of immersion rod (say a power cut happened and you forgot) water will continue to boil until its all gone, and then the rod will melt off. (happened to me once )
(b) Chances of electrical shock are more specially if you have elderly/kids around.
(c) No scope of shower bath.
Regards,
Mohit