The "popular times" thing here in Google has Sunday being even more busy than Saturday, which is opposite to what I expected. How busy would you think it would be on a Sunday in March?

If you arrive early on either day that would be a great benefit to you.

We went when it opened on a Saturday in March and had the zoo to ourselves until about 1130/12 ish

Saturday and Sunday are quite popular anytime of the year, but it will be much less crowded in March then it would be in May or June. I hope I was able to help you somewhat in your decision.