ZRod

The near instant hot water is great, being able to let both kids take a bath separately and then take a shower with hot water still available is why I went tankless with CNG.  The wife has her big tub and can actually have hot water the whole time and the warranty was double our old stand up water heater.  I have heard some pipes don't work with them but we built our house in 2015 with the right piping for it.  I recommend it if you have a large family.

@ZRod Your existing furnace flue pipe may be large enough to handle the additional BTU’s of a gas water heater. Roughly, 4” or smaller won’t be big enough, 5” might be big enough and 6”+ likely is large enough. A HVAC guy should be able to tell you how large of a diameter flue is needed. Then you would know if gas is even an option. The little bit of vent piping from a WH to furnace usually amounts to nothing but, if the main stack isn’t big enough, I’d forget it 

If you can add more electrical service/panel room reasonably, I’d still try for an electric tankless over an electric tank. Just my preference, I don’t like electric tank heaters.

I installed a natural gas tankless in my last home and loved it, but I went with a gas tank water heater in my new house. There are pros and cons to both setups but I found that I wasted too much water waiting for the tankless to start heating and for my lines to clear the cold water. I have a tank system with a hot water loop now so I have almost instant hot water anywhere in my home.