The finger width is really just a matter of preference. I think the narrow finger works better and produces a more professional result when working with small wires like are used for security. If you're running coax or speaker wire through them, the wide finger fits better.
Best thing about the narrow finger stuff is that it's easy to break off the fingers (they are pre-scored to facilitate this) to make wider slots!
The 1.5" x 2" is a great fit for most structured wiring boxes. The smaller stuff will work but provides much less room of you have lots of wires to stuff in there. I'll often leave "service loops" which basically means to leave the wire long. I can double it up in the wire duct where it can't be seen and then the wire is still long enough if it needs to be re-routed somewhere else in the box down the road.
The Panduit stuff is not inexpensive... I just purchased a case and it was over $450 wholesale.
If anyone on the message board is interested in short lengths, let me know. I'll sell the 1.5" x 2" by the foot...