Well the premium on GB has Magna Slings on it which is pretty cool.

The Pros have mechanisms removed to cost reduce them, so they have less "wow factor" in general. USUALLY the Premium is better, but sometimes the Pro will do.

In the case of Ghostbusters, the Premium is the one you want. The slimer moves much more than the pro, which just bobs a bit, it has magna slings which make the balls do crazy things once you power them up, the premium has a scoop on the right that is missing completely on the pro and replaced with a kickout, the ramp on the premium goes under the playfield then back up unless you lock a ball in the subway (the ramp and ball lock is crap on the pro and nowhere near as cool), and the trim like the library is a full 3D building on the premium, but just a flat piece of plastic on the pro. You get the idea. Pretty major differences.

Stern usually releases a feature matrix that shows the differences, but sometimes they get them wrong, or something that seems not a big deal on the list turns out to be more important than you thought. Anyway, here's the feature matrix for Ghostbusters (note that when Stern says "butyrate" they mean a crappy flat printed plastic piece):

http://www.sternpinball.com/upload/games/ghostbusters/le/Ghostbusters%20Features%20Matrix.pdf

And sometimes the Premium can have features that end up with scoring advantages over the Pro. This is the case with Aerosmith which has a double scoring mode on the upper playfield you can't get on a pro (it's different than 2x scoring, and stacks with that on a Premium/LE).

A lot of it comes down to personal preference. You can find someone that prefers the pro over the premium on any game.

If it’s a modern Stern just look up the feature matrix and you can see the difference in features and art/trim. Then go play or see both in person and decide if the cost is worth the extra money or not.

Game play between the pro and premium can be a little different because of missing or added features....it’s really personal preference and budget for most. Many get the pro because it’s what they can afford or they can’t justify paying more for the extra features on a Premium or LE which in some cases are not many and the value is not there.