We always rent a car - cheaper, easier, quicker, and less crowded than getting the cruise line's transfers. Definitely cheaper than a taxi. Plus, it allows you to do some sightseeing or shopping as you see fit.
Hint; if you've never been to Cape Kennedy, I highly suggest getting a late air reservation back home and spending some time there after the cruise. It's an awesome place to visit and it's just up the road a few miles from Port Canaveral.