I’ve had a gas unit for 7+ years (Takagi brand). I did cost more initially, but I recouped the investment after about 5 years of use, as my monthly bills went down significantly - both because there is no pilot light, and because I only heat water when I need it, rather than keeping a hot-water supply constantly hot.
We absolutely LOVE it. You never run out of hot water, which is great when you have a house full of guests, or have a small window of time in which to have multiple people shower, do some laundry, and run the dishwasher.
I would not consider electric - as others noted, too long to heat up, plus more costly than gas for the same heat/volume output.
When installing gas, the plumber had to run a 3/4” pipe to supply the fuel (regular gas line, as with a stove-top or standard water heater won’t cut it). Plus there was a double-wall stainless vent pipe required. I also installed a pipe for outside air to feed into the combustion chamber, rather than drawing from the garage utility closet.
My natural gas bill went from $30/mo., give or take, to under $20/mo. during the summer months (when I’m not using gas heat). No real way to tell what the savings were the other months, but I assume they would be at least equal if not better.
If there is a down-side, it’s that these can only heat and deliver so many gallons per-minute, so using 2 showers plus another appliance (washer, dishwasher, etc.) will mean colder water than desired. Make sure you spec a unit with the highest GPM (gallons-per-minute) rating you think you’ll reasonably need.
Right now my stove uses a 5/16 flex copper line to feed it ... about 55 ft from the source.
We live in a mobile home so the piping is that PEX 1/2" plastic crap .. very easy and fast stab coupling to work with.
I don't THINK I need to change the water lines.