Most manufacturers make a series of "good-better-best" models, and one of the featues is often more insulation. But the difference is not huge.
A basic gas WH might have an energy factor of .58 where the best one might be .62
Gas WH have a somewhat lower energy factor than electric because heat is lost up the flue pipe when the burner is running, and also to some extent by convection during standby.
You have to go to a power vent to get better numbers.