You got lucky 63tb, I've worked for a major Cruiseline for 14 years, I can probably kinda help you out...
First of all, the itineraries... The only difference between an Inside passage cruise and a Gulf cruise is that one goes from Vancouver and ends up in Anchorage (or somewhere near there) - This would be a gulf cruise and the other goes back to Vancouver (or San Francisco, or Seattle etc.) (inside passage)
The Gulf cruises can be done northbound, or you can fly up to Anchorage and sail southbound... Normally the same towns and such, just in reverse.
For a first trip, its not going to be critical which itinerary you choose, you're going to get glaciers, neat little towns, totems and all that regardless.
The Gulf cruises are especially usefull if you're planning on taking a tour into Mt McKinnley, Fairbanks or something of that nature because they put you in position up there.
If you've never cruised before, your best bet is to see a travel agent that specializes in cruises, they will be best able to answer your questions (you've likely got many of them).