I haven't sailed on MSC but of the other three, I probably prefer Carnival as long as it's not school vacation so that the ship isn't overrun with kids. Celebrity, Princess, and RCL are all nice, too, for different reasons - Celeb for very modern decor and interesting food choices, Princess for a quality relaxed traditional cruise experience, and Royal for entertainment. I like Carnival for its festive vibe, no reservations required for entertainment, and great free food choices. Drawbacks: Celebrity has lots of upcharges and they promote them relentlessly, Princess can be a little staid, and RCL's Oasis and Quantum classes can be overwhelming with reservations being required basically for everything you want to do. With Carnival...there's the kids, not the drunks, there are no more drunks on Carnival than any of the others. In the end, they're all more similar than different - the ocean is the same for everyone, and they all go to the same ports in the Caribbean.
I suggest, though, that you go for a 7-day RCL cruise, it will give you a better idea of what the line is really about than the shorter Indy cruise, which will be more of a party boat just because it's a shorter cruise.