This is called 'torque converter creep'. Even though you consider your '99 to be inferior, it actually demonstrates that it has a better, more efficient torque converter design,at least at idle.
I am assuming here that the engines are the same (3.8 V6?) and the transmission is the same 4 speed automatic, and neither engine or transmission is malfunctioning.
A torque converter is the device that allows an engine to idle when the car is stopped, but 'engages' and moves the car as you step on the gas. Converters are hydraulic devices and thus place a certain amount of drag on the engine.
This drag is what prevents the car from rolling back.
Also, the greater the drag, the more fuel is used to for the engine to idle when the car is stopped.
Therefore, since the '99 has less drag, it is probably more fuel-efficient at idle.
Look here for more info:
http://en.wikipedia.org/wiki/Torque_converter
Also, it is possible your '99 has a 'looser' converter, that is, one that allows the engine to rev a bit higher than the '95, when accelerating. This makes for snappier acceleration at the expense of a slight decrease in fuel economy, at least until the car reaches cruising speed.
Personally, I would consider the '99 to have a more favourable set-up, despite the hill problem you have.
Also, I think you should check and compare the idle speed of the '95. If both cars have tachometers, do they show the same RPM's at idle in gear? If the '95 is idling higher than the '99, it would 'creep' more, but also may have a problem, such as a vacuum leak or bad IAC (idle air control motor).
Finally, it would be difficult to give the '99 the same 'creep' as the '95, without changing the converter, which requires a transmission removal.