Hello Nina,
I first took our daughter to Disney when she was three years old too. Your Princess is going to love it, I just know it!
You can have a successful trip, even with a little one! My family has traveled with toddlers, so I know there is so much to do and see at Disney World. There's Dumbo, The Many Adventures of Winnie the Pooh, It's a Small World, and of course, you can now meet Mickey Mouse at the Town Square Theatre. And that's just in the Magic Kingdom! Imagine how the list would look if we looked at the other Parks!
I have never gone on a Disney Cruise (although it's on my to-do list!), but I know fellow panelist Leslie has brought her toddler on a cruise and is planning to do so again. They offer so much for kids of all ages, like the character experiences and fantastic live shows you would expect from Disney.
But why try to decide between the cruise and Disney World? Why not do both? Disney offers land and sea packages too. But if you have to choose between one or the other, honestly, I don't think you can make a "wrong" decision. Cruise ships can be expansive, especially for a preschooler. So no matter what you choose, just remember, keep with your Princess' schedule. To this day, we still take quick trips back to our resort for midday breaks. And don't feel pressured to stay out late. Head back earlier in the evenings. That way your Princess can get to bed as close to her regular bedtime as possible.
Hope this helps!