We just got back, so I'm not sure how helpful this will be.
Mon 2/18 - IoA - empty! 5-10 min wait for most rides (Hulk, DD, etc)
Tues 2/19 - Studios - a bit busier but not bad - Mummy 5-10 min wait,
Disaster 15 min wait, MIB walk on
Wed 2/20 - US/IoA - same
Fri 2/22 - IoA/US - much more crowded! Up to 40 min waits
I thought about purchasing Express pass for 1 day before we left home, but decided to wait until we were in the parks. The Monday pass was most expensive (online) and the Friday pass the least $$. I figured Fri would be the least crowded day and Mon the busiest. It was the complete opposite!
My best piece of advice is to get there before the parks open!! The first hour is fairly quiet and just ride all the Big rides as many times as you can. Also, use single rider line if it's open.
I'm not sure when spring breaks begin in March, so it's hard to say how crowded it will be when you're there. I offered the Express pass to my boys (ages 12 & 14) for one day and they didn't feel they needed it. This was during Presidents' week which can be very busy.
Hope that helps!