We have been to WDW several times President's week, twice over Christmas week, never over spring break so I can't comment on that week. It is definitely crowded President's week, but I don't think the crowds can compare to Christmas week. I have never seen so many people in my life as at MK on NYE. (I love WDW at Christmas time - don't think I'm bashing it.)
We love President's week because the weather is generally good (sometimes a little cold, but if you wear layers you can handle the weather), and 60 degree weather actually feels pretty good after February in New York (you're from LI too - I'm sure you would agree!)
If you get to the parks early and make good use of fastpass, you can do everything you want to do during President's week with no problem.