• Princess Grand Class ships have Skywalkers- a nightclub that blows away anything Carnival ever thought of having
• Princess does a better job of breaking up huge, open deck space into smaller more intimate areas
• Carnival has FunVision, an in-cabin interactive system that lets you view your shipboard charge account, buy shore excursions and watch pay-per-view movies
• Princess has better, sometimes excellent food but lacks consistency
• You can buy cartons of cigarettes on Princess and they hold them until the end of the cruise, on Carnival you can have them when you buy them
• You can buy bottles of booze on Carnival and they hold them until the end of the cruise, on Princess you can order bottles from room service and have them right now
• Carnival has an incorrect reputation for being a big drunken party all the time (not any more), Princess has an incorrect reputation for catering to boring old people (that baton has been passed to Holland America, right or wrong)
• Carnival has a better kids program- Princess' is good but inconsistent from ship to ship
• Pricing is competitive, comparing numbers to numbers Carnival will come out less expensive but Princess can be a better value
• Princess ships all seem to have a similar layout and decor, each Carnival ship has a different theme
• Princess rules embarkation after you get into the terminal, Carnival is trying but still not there
• Carnival rules embarkation line management , Princess seems like it's the first time they've ever done it, every time we've boarded.
• Carnival has the reputation for the best shows, I rate both lines the same
• Princess sails to more places
• Carnival management is totally accessible, good luck emailing somebody important at Princess
• Princess employs more muted tones and promotes a subdued but semi-elegant ambiance, Carnival is very glitzy, Vegas style with lots of neon on older ships, not so much on the newer ones
• Stand in the middle of the atrium on any Grand Class Princess ship and you might not be sure which one you're on, Each Carnival ship has a distinct theme and individual personality
• After trying both lines I feel like Princess has more moaning, complaining passengers. Carnival's passengers are more there to have fun. Age is not a factor in either group.

What's a good way to remember which side of the ship is Port and which one is Starboard?

On Princess that's easy.  Chances are there is  different color carpet to help you remember.  This really helps when you first step off the elevator. Here's a catchy phrase to help you remember:   

Port wine is RED and should be LEFT alone
Red being the colors on the left side of the ship as well

If you pay real close attention to other ships at sea you'll notice a green light on the starboard side and a red light on the port side of the ship.  That lets other ships (and us too) know which direction another ship is headed- important information if you are on an iceberg and see the Titanic in the distance.