This is an interesting question:
Here is my take on it... having been on both ships in 2004.
Both are decent ships. The crews on both were excellent.
The Dream is older and smaller, and was NOT designed to support "freestyle cruising" as it pre-dates that concept. You cannot really get the true benefits of freestyle cruising on the Dream or it's sister the Wind. There is only so much you can do with a smaller ship that was originally desgined to accomodate traditional cruising. The deck plans are a little cludgy, and you find yourself going up and over certain decks and then back down to get around the dining rooms and/or Stardust lounge to other common areas.
The Sun is newer and larger, and was built to support the concept of freestyle. As a result, I would say that under the way NCL operates today, using Freestyle as it's basis on all ships, that I would prefer the SUN over the Dream. The Sun gives you many more choices in terms of dining and can handle freestyle cruising much better as a result. The deck plans are laid out such that it is much easier to traverse the ship. The common areas are wider and more expansive, and you feel less compacted aboard the Sun.
So... if all things are equal (itinerary, etc) I would prefer the Sun, and of course the newer ships, the Dawn, Star, and the upcoming Jewel.