Keep in mind as with any profession, not all travel agents are created equal. Plus, when booking a cruise, you really should be talking to a Cruise Specialist. Unfortunately, alot of agents will claim to know about cruising, but you'll find they either haven't been on any or have only been on a couple. But it's still better to book with an agent than booking with the cruise line or an online site.
As for the choice of these two ships; that's really a tough one because I'm going to show my prejudice on the matter. Between the two, there is absolutely no doubt about it - the Celebrity Solstice-class ships are my overall favorite. We've been on 43 cruises, including 41 ocean cruises and we like the Solstice-class ships of all of these cruises. The food is outstanding, the ships are great, and the service is wonderful.
However, as I mentioned earlier, Celebrity draws an older crowd. So it's rather sedate and nightlife can sometimes be a bit lacking for those who are more active.
While I know you would absolutely love Celebrity, because of your age and what you are looking for in a cruise, I feel you'd find Princess very much to your liking. While the food is not quite up to Celebrity's quality, it's still very good. The ships are great for a younger crowd and the crew has always been wonderful on the many Princess cruises we've been on. Our children will testify as to how good Princess is - they all talk about our last family and friends cruise, and the wonderful time they had. (They were 27, 23, and 21 at the time.) The two things they really enjoyed were the fact all the swimming pools are fresh water (unlike many cruise lines) and the night clubs stayed open late. In fact, one night they were partying with the ship's magician, cruise director, assistant cruise director, one of the head chefs, the ship's photographer, and a bunch of other crew. When they went to close the nightclub at 2:00am, our little group of party people talked them into to leaving the club open for another hour.
And yes, most cruise lines will try to seat people of similar demographics together. Best advice on this is either to select a large table for dinner or select anytime dining. With anytime dining you can decide to go to dinner whenever you want and you'll be sitting with different people every night.