My totally uneducated opinion would be hot.
In my house the run from the tank to the tap is pretty long, so if you want a cup of hot water you have to run the hot for 60 seconds before you get warm and 90 secs before it is really hot. Getting a cup of water forces the water heater to heat 6 gallons.
What you are saving is that 90 seconds of running and the time it takes to warm up those pipes so the water is hot. The run time to get full heat drops to maybe 10 seconds and 1/2 gallon and though you heat the water twice, you heat way less than twice as much of it.
When you're filling the tub or running a lot of hot for dishes or something, you will wind up drawing from the main tank and get lots of hot to work with.
I have no idea whether it is actually kosher to feed hot into the inlet of a small electric tank, so that's something I'd be interested in.