If you are going with someone who has at least a Preferred Annual Pass they could get a pretty good discount on a room and everyone in the room would get unlimited Express Pass, plus they could get you a discount on the park tickets. I think the discount might be only like 10% though. The Express Passes you purchase are good for only once per ride, whereas the Express Pass you get for staying on property are unlimited, meaning you can ride each ride over and over if you want. Just remember that express pass can not be used for Forbidden Journey or Pteranodon Flyers though and that you don't get Express Pass at the new moderate resort (Cabana Bay). Also if you are staying onsite you get early entry, which will be very valuable during spring break to avoid some of the crowds.
We're going to be down there March 21-31st and even though we have a condo rented for that time, we are still staying one night onsite at Universal, just for the unlimited Express Pass and early entry.