What do you want to see and do on your Alaska cruise ?
How much wilderness do you want to see ?
How much time do you have ?
If you think you'll be cruising again to Alaska, then IMHO, it doesn't matter. Try a one way and next time try a round trip.
Not sure if I would ever cruise to Alaska again, for my first Alaska cruise, I took a south bound, Seward to Vancouver with a pre cruise land tour (4 days) (Denali National Park) - I'd have to say that it was great. If all the time (and $$) you have is to see the coastal port towns (skagway, ketchikan, juneau...) - ok, they're really great, but if you've got the TIME, there's more to see in Alaska's inland. Spend time (several days) inland to see Mt. Mckinley, Denali and other Alaska National Parks. I think the north most port (for round trip cruises) is Juneau - can you take a flight to Denali to see Mt. Mckinley ? Is there enough time to get out of the plane and take a land tour of Denali National Park ?
Of course, lots of people visit California all the time and see only the coast towns: San Francisco, Monterey/Carmel, LA, San Diego - the coast towns and never see Yosemite National Park.