We have had both mini suites and balcony cabins on the Princess line. The balcony rooms themselves are nice rooms. They are large enough for 2 people and would make a nice room for even a regular hotel. The mini suites are 50% larger than a balcony room. If you look on the ship's deck plan you will see that 2 mini suites take up the space of 3 balcony rooms. Like the difference between coach and first class on a plane. The suites are twice the size of a balcony room. One suite takes up the space of 2 balcony rooms.
What you get is more sitting room, another TV, a tub and shower instead of just a shower and more closet space. Everything else is about the same.