I'm sure others will chime in, but here is a start.
The Premium package included things
Traction Control
ABS
Full instrumentation dash (Water temp, battery, fuel & oil pressure as opposed to just fuel and temp)
MyColor dash
There is also interior trim enhancements (chrome like trim surrounds on the vents and aluminum on the dash)
Not sure if leather is included ot if that's seperate
Pony Package is mostly an exterior appearance package that includes the smaller foglights and the GT Bullitt style wheels, although I believe the suspension is slightly upgraded with the GT springs and sways.
Not sure if the Premium package is a requirement for the Pony package.
I have seen GTs that did not have the Premium package, and only hade the base gauge cluster.
I have a Premium, that didn't come with the Pony Package.
My car has ABS, Traction Control, Leather, full gauge instrument cluster, MyColor guages, and the upgraded interior trim that has the chrome plastic around the vents and the brushed aluminum look on the dash.
I have since added a GT style grille with the larger fog lights.