I have played with two models of gas tankless heaters and I find I can't get a low enough flow rate for darkroom use. At low flow rates the burners turn off, giving you only cold water. I imagine the electric ones may be better in this regard, but I haven't tested them. I now use a heat pump water heater (50 gallon tank) for the house and darkroom. It's much cheaper to run than the propane tank heater. But that's not going to be worth the expense for a heater that's only used for the darkroom.
Larry, the cold water at low flow rates is my biggest concern. I wonder if the electric ones have the same issue. I am digging about on the home renovation forums but haven't found my answer yet.