We are at Universal now, just finishing up our first visit here. DS14 and I had a great time! It was definitely different than Disney, but not in a bad way. There aren't as many kiddie rides, but with kids your childrens' ages, I think that will be a good thing. There are a lot of rides that will be fun for the whole family, like Shrek, Jaws, Men in Black, Sinmpsons, NIcktoons and then there are some great coasters if you are into those.
I was really concerned about there being no fast passes(could not pay $50 each for Express and we did not stay on site), but we just got there t opening and started at the back of the park(Simpsons) and had very little wait for the first hour or so. We also discovered that the single rider line was a real time saver on rides like Men in Black, SPiderman and Revenge of the Mummy. With 2 adults and 2 kids, just send one adult, then the kids, then the second adult. That way, someone is always with the kids except when they are one the ride. Truth be told, DS and I were in the same car 1/2 the time, just in different rows.
(just saw you have FOTL, but you still might consider single rider at times. Seemed like the single rider line moved about twice as fast as the express for the MIB ride)
If you get there early, you can get most of the rides in the 2 parks done in one day. Try to do the most popular rides early in the morning and late in the evening. I think two days would be better, or maybe just one full day and another morning or evening.
Marsha