Calena,
Over the course of our upcoming two-week trip in March, we are also staying offsite and plan on spending most of our time at WDW.
However, based on our really positive experience in March 2005 during March Break, we are once again booking a one-night stay at the Royal Pacific Resort at Universal Studios. By staying onsite you get front-of-the-line access to all the rides for two full days (it's like unlimited fastpasses).
We love the resort, and will get to walk on to every ride for the duration of our stay, but use their gorgeous resort pool. The experience completely spoiled us - don't get me wrong, I love fastpasses at WDW - but to be able to go back to your favourite rides over and over again with no wait time was amazing. I weighed the cost of a night at the resort (we were able to get a CAA discount) vs. wait time in lines and it was a no brainer. Just something to consider if you do decide to try Universal and you could spend a night onsite.