You are looking at 2 totally different routings, Carnival is one way- Vancouver/Whittier and the RCI ships are round trip Vancouver. It make a big difference on flights and if considering a one way- add days to tour the interior. Comparing the two- neither goes to Glacier Bay, the Carnival has 4 ports and only College Fjords. RCI has Misty fjords sailing on the Seranade- DON"T be having dinner during sailing- get out on deck for the great views.
There are no party ships yet sailing Alaska- so late night activities are less than what you will find in Caribbean sailings. For me everything I go to Alaska for is OUTSIDE the ship and early mornings can be a big bonus. Be certain to budget for costly excursions- you get only half a trip without them.
As for air- I never use cruise air myself and always book direct. Compare options- and factor in all transfers- included with cruise air. Allow $25++ cab from Vancouver airport and if going one way- at least $59pp for the Alaska RR Seward/Anchorage. With the one way- look under "multi city" option on flight booking sites- you don't want one way tickets. Once you narrow down your choices repost for further information.