I run both when initially turning on the heater from cold. After it get's hot, I'll turn the gas off. I flip both on again in the morning when taking showers so it will re-heat as fast as it can. After the showers, I turn the gas back off.
I primary run electric when we are at a campground and have electric hook up as the electricity is paid in the cost of the campsite.
At home, if we run hot water, I use gas and do not use electric. I've never run the math on it, but I just feel the gas is cheaper than my electric bill.
When traveling on the road, it is always off, completely.