Hi Robbin!
Well Robbin, if you are looking strictly at crowd levels, both times are considered to have the "highest attendance" by those of us in the know. However, I find the crowds to be somewhat lighter towards the second half of the month in August because of students returning to school.
That being said, I much prefer to visit Walt Disney World in March, rather than August. The crowds won't be that much worse and the weather should be gorgeous.
Epcot's Flower and Garden Festival occurs during the month of March and it is truly a sight to see. The 2009 festival begins March 18, 2009, so you will be in for a treat! As you mentioned, March will be here before we know it and you really need to start planning now to get the most out of your vacation at WDW.
No matter which time you choose, I'm sure that your family will have the time of your lives!
Much pixie dust,
Jackie S.