So much of budgeting is predicated on when you will be going, how long you will be staying and what kind of accommodations you will need. Instead of giving you an approximate number that would probably be dead wrong, I'll just give you some advice on what to keep in mind while you create your own budget.
Since you have an airline credit, you will pretty much be limited to their flights. Try airfare search engines like sidestep.com or kayak.com to research airfares. Both can give you an idea of the prices to expect and kayak will also let you know whether fares are expected to drop or rise.
I would avoid any "spring break" dates for travel not only because the parks and resorts will be more crowded but also because the prices will be higher for rooms and airfare. So anytime from President's Weekend until the week after Easter would not be a good idea. I was in WDW last year from Jan. 3-10 and it really was a great time to visit. It's value season, the parks are relatively uncrowded and the holiday decorations were still up for the first few days of my visit. The weather was very nice; warm but not hot and very little precipitation. The lone drawback is that the parks close earlier but since they weren't too crowded, we were able to do everything we wanted to do.
Are you planning to rent a car? If you're under 25, it could be very expensive. Car rental agencies do not like the under-25 crowd and they charge a hefty premium. If you're going to need a car, I recommend using hotwire or priceline to bid for one. If you're going to borrow your friend's car (or if they are going to be your chauffeur), you should offer to chip in for gasoline and pay the parking fees ($14/day at either Disney or Universal).
Choosing between Disney or Universal is really a matter of preference. Disney will be more expensive but has more to see. Universal has a 2-park pass that is good for 7 consecutive days. It runs less than $110 w/tax on some ticket reseller websites. A 7-day MYW adult base pass at Disney is more than twice that amount and wouldn't let you hop. But if you sign up on the Disney website to do volunteer work you can earn a free 1-day ticket for the park of your choice. I would recommend doing this if you want a day in Disney on top of some time spent in Universal. The drawback is that Disey doesn't have all their ducks in a row for this one yet and you would have to wait until later in the year to do your volunteer service and receive your free ticket certificate.
Then there's the issue of food. Staying with friends means that you will at least have access to a kitchen but you'll probably want to chip in for groceries. If you're getting a hotel room, you might still want to keep a few groceries in the room for a quick breakfast or may a snack when you get back to the room in the evening. You'll also probably want to plan for a couple of meals out and perhaps some bar hopping or clubbing.