Of the 2, I would say visit the Zoo. It's easier to get to than the Tierpark, being right in the centre of Berlin. Aim to spend the full day there, otherwise you'll be rushing around trying to see everything. The hippo house is a must, but also the area round the Bird House is fantastic if you are into birds. Kagu, Ocellated Turkeys, Andean and James Flamingos, Pied Hornbills, Great Curassows, Channel-billed Cuckoos, Kiwis (although we didn't manage to see them). The bird of prey collection at the Zoo is also very impressive. Although, having said that, the Tierpark does have a pair of Harpy Eagles.... Whichever you choose, you'll have a great day, guaranteed.
Just noticed the date of these posts, you'll be in berlin as I'm typing this, so just enjoy yourself!