It's busy. We've pretty much sworn off October since we went in 2016 - it's just too crowded for our liking, especially the second week.
That being said, many people do it happily, but they're usually more experienced Disney travelers. You must go into it with fastpasses, a good plan, and be there at park open. As long as you have realistic expectations, you will do ok. But expect it to be busy.
As for March, that really depends on when your break is and when Easter falls, because that can be a very busy time as well.