While both places will be too crowded on Mother's Day, This is the season of the Mummy in NYC. The Egyptian Galleries in the Met now have a scientific Mummy counterpart at Natural History (and spoiler alert, this exhibit includes Mummies from Peru - who knew and no pictures for good reason). And, the Mother female IMAX humpback whales are pretty amazing!
Membership has its privileges at Natural History. It's really nice to order your tickets for special exhibits and films and pick them up from the machine when you arrive.
While at the Met, when you've finished with the mummies make sure you visit the Rooftop and any other exhibit that interests you. (Photo has received over 2500 on Twitter. Amazing for me.). The Met has a suggested admission fee. If you don't pay the full fee, I encourage you to spend the difference by shopping, eating, or drinking during your visit.