You only get about 70% of the WH contents before it cools off a lot from the incoming water. So, with a 'normal' showerhead at 2.5 gallons/min, that's only about 8-minutes, depending on how hot you run the heater - you'd get longer as you turn it up. That's enough to get clean, but not enough to relax much or sooth some sore muscles.
If you do run the thermostat up high (it has some benefits - you won't grow nasty things in the WH), then you should install a tempering valve to limit the max output so someone doesn't accidentally scald themselves.
Efficiency of the burners probably isn't all that much different, but insulation may be better on the new units.
There are some that are more efficient than others.