Energy Usage, Conservation and Weather Stripping - Question about lowering electric bill: Windows or OnDemand Water Heater?

I have a 1928 rowhome which is located in the middle of the row. There are 9 windows ranging from 2 wooden single pane windows, 1 metal single pane window, 1 small window which has the glass slats that close, and 5 very old vynil dual pane windows.

My house has natural gas, uses a boiler and radiators for heating and window air conditioner units (1 12,000 BTU window unit for 1st floor, 13000 BTU portable unit for basement, 2 6,000 BTU for the bedrooms) it has 2 bedrooms, 1bath and is 3 stories (including basement)

I like cold better than hot, and my electric bill is always more in the summer. I keep the A/C anywhere b/t 60-65 and same with the heat.

My question is will I see more savings by replacing all windows with a better, more efficient dual pane window or by replacing the water heater (fairly new 40 G tank - natural gas) with something like the Rheem Tankless Water Heater RTG-53PVN. I do like to take showers, and I am the only one living here (i do tend to take long showers -- until the water runs cold).

Thanks for your input, please feel free to ask for more specifics.

Windows and insulation....
And get newer window units or go with central air..

So you have 4 single pane and 5 dual-pane windows. What si wrong with the dual pane vinyl ones? Are they warped, ill-fitting, felt worn off of the sides, ?.

After reading in your post that you'd rather be cold than hot, I thought that if you took cooler showers instead.......:D

Changing all the windows would pay for themselves in very short order for both heating and cooling.