The big advantage for going one way (for me) was the opportunity to spend time inland and see something besides inside passage towns. The Kenai Peninsula is beautiful and the roads are in great shape. Anchorage is worth a visit and Denali is an opportunity you shouldn't miss!
The round-trip advantage of Vancouver was the shorter flying distance, especially if you live somewhere other than the west coast. LAX to ANC was around 5 1/2 hours.
I don't recommend any one way trips without at least 5 additional interior touring days. It is a big expense in time and money not to. All cruises do inside passage. Choose carefully your ports, glacier ( Glacier Bay my clear preference) and time in ports. Additional factors for me is price and date- really like going mid to late August. BUT do not consider a round trip- a sloppy second- it's a great trip for the time and money!!!