A couple of things. Yes you can of course search the Costco site. You will likely get a good price there.
You might, for your first cruise, want to talk to a travel agent who specializes in cruises. The TA can offer more personalized service and ideas. You usually don't pay extra for the service as the cruise line pays them a commission.
A good TA will watch if the price drops or a sale with more perks comes up before final payment due date and get the price lowered for you. I'm not saying Costco is bad, but it will be a bit more self-service.
A good TA will share with you what to expect on boarding, dining and beverage options, etc.
A lot depends on you and what you like. A good TA can get to know you and make a recommendation on the best ship for you.
Examples of what a good TA might ask:
- How old are the kids? One line has more teen-oriented stuff, although both will have children's programs.
- When you travel do you prefer to stay at a budget motel, a Hilton, or a Ritz (as example)--if you are budget person you may be happy on Carnival. If you are a Ritz person you will not be happy on Carnival
- Where are you coming from? If flying in you may want to stay overnight the day before to make sure you don't miss your sailing (it happens). A TA can help you with these questions and more.
There can a big difference in the experience between RCCL and Carnival if those are your choices!