There are so many things to weigh up when selecting a stateroom. For first time cruisers it is difficult because most don't have an idea what to expect when they get on the ship. One thing is to look at the different category stateroom layouts (Princess.com) to see if you can/can't live with any of them. I remember our first cruise and we had a balcony stateroom that was a total waste. We were so excited about being on the ship and all the activities going on we used the stateroom for sleeping and getting clean. I doubt we spent more than one-hour total on the balcony the entire trip. The trip would have been just as nice with an inside stateroom. Now we only book balconies or mini-suites because we spend more time in the room and on the balcony because the onboard ship activities are not as exciting anymore.
All staterooms have there good and bad points. An inside is the least expensive but has limited storage and you spend a lot of time bumping into each other due to limited size. I would never have more than two people in an inside stateroom. Outside staterooms with a balcony are very nice, bigger, have a little more storage but more $$$. We like the mini-suites the best because they are bigger yet (twice the size of an inside), tons of storage and best of all it has a tub and shower. The shower in the inside to balcony categories are about the size of a phone booth. The last thing you want to do is drop the soap!!!! My wife thinks the tub and shower in the mini-suite is worth the $$$ by itself. Only drawback to the mini is the cost, which is usually double the cost of an inside.
You really can't go wrong with what you select on your first cruise. One thing to think about is if anyone gets motion sickness easily you want to book a stateroom near the center of the ship on one of the lower decks. This is the area of least motion. Don't worry though, it takes some real strong seas to feel any motion on the Sapphire/Diamond because they are so big!!! Avoid getting a stateroom near any elevators or main corridors leading to/from the public areas (less foot traffic = less noise). We have been on the Diamond and Sapphire ships and both are wonderful. You will have a good time no matter which on you choose......
Good Luck!!!!