This is a much asked question.
First some basics: Marc is not correct about Glacier Bay. Both Princess and HAL offer Glacier Bay on RT itineraries. I would seriously recommend Glacier Bay for first time Alaska cruisers. See Alaska Glaciers
for my thoughts on glacier viewing.
Airfare to or from Anchorage on an open jaw is not a whole lot more expensive than RT to Seattle or Vancouver. Your travel agent can advise you. If you sail from Vancouver, many flights will take you to Seattle and you need to arrange the transfer to Vancouver which will be an additional expense. But in any case, expense is really not the biggest consideration.
Most of the established cruise ports are on the panhandle of Alaska in the Inside Passage. So when you continue on to Anchorage (all these remarks apply to the opposite way Anchorage down also) what you basically get is College Fjord and the Gulf of Alaska as opposed to more time in the inside passage.
One of the major reasons for the one way itinerary is to connect with cruisetours. If you want to go on to Denali and Fairbanks, or even a longer tour, then Anchorage is the best place to connect with the tour. If you are not planning to add a tour to your cruise, it really does not matter. Check out our trip report and pictures at Alaska Cruisetour, Fairbanks to Denali
for some cruisetour information.
So, ultimately, I don't think that RT or one way is the main decision. I would suggest you talk to a good Travel Agent about the ships available and let the ship, the ports, the dates, and the bottom line prices make your decision. There are so many variables in planning a trip to Alaska that only the very brave would do it online or via an 800 number instead of visiting a good face to face full service Travel Agent in their home town.
You can check out trip reports of our 8 trips to Alaska, including both one ways and round trips and also a cruise tour at Cruising with the Halls
Also there is a dedicated Alaska Board and that would be a good place to post this.
Have a GREAT cruise!