When purchasing a tankless water heater, the things to consider are gas vs electric.
If you only have electric, your flow rates and temperature variations will be a factor. If you have gas, the gas water heater will have a higher flow rate and a lower inlet temperature. A gas instant water heater can take the water from an inlet temperature and raise it 65 degrees at 3.5 gpm, which is enough for 2 showers at the same time. An electric can raise the temperature 50 degrees at around 2 gpm flow, enough for one shower at a time.
An electric instant hot requires no vent, and is a safer (though less efficient) water heater. It will last longer than the gas version and cost less for the installation. It has a 20 year life span. Remember for the electric instant hot, you have to have a 225 amp service.
The gas water heater is more efficient over its lifespan, but has a higher installation cost for the venting and flue. It has a 15 year life span with regular maintainence.
If you average the total installation cost and the cost to operate the gas and electric units, the cost comes out fairly equal.
The difference is recovery time, though you can probably install two electrics for the same price as the gas, but you will need a huge electric service.