Most places, gas is considerably cheaper to operate than electric, but that may not be true in your area.
A gas tankless needs a 3/4" gas supply pipe. You probably don't have that available right now at the garage location, so there would be the cost of running that new pipe. Also, they usually require a stainless steel flue thru the roof, adding several hundred dollars to the first time install.
In your norther climate, most of the electric tankless companies just recommend NOT. First, you will need a minimum 120 amp circuit to the water heater, as opposed to the 30 amp circuit you have now. Second, the GPM at a temp rise high enough for a good shower or bath....will be much smaller than the rating that you will see advertised in the large print. Check the fine print!