I've been to WDW during both of those months, and I believe there are advantages and disadvantages to both.
In general, of the two months mentioned, I'd recommend March. The crowds are fairly large to be sure, but not as large as in July. The weather is better in March, not nearly as hot and humid as in July.
And, of course, most children are still in school in March, except for the considerable number of ball players and cheerleaders who tend to decend upon WDW in March, making plenty of noise and having plenty of fun. (They don't bother me at all, by the way, probably because I spent 30 years teaching in a public high school and love kids that age.)
Also, the Atlanta Braves baseball team has their spring training games at WDW in March, but it seems to me like that has very little impact on theme park attendance.
Either way ...
Enjoy.