I didn't know there were two! Twice the fun
Cool city to grow up in!!
Yes Berlin is cool in general but zoo wise and animals wise it's really great. And both zoos are the main reasons for gaining my interest in animals and zoos. Maybe I wouldn't have gotten these interests with out Zoo (
https://www.zoo-berlin.de/en) and Tierpark Berlin (
https://www.tierpark-berlin.de/en/).
But caution! There is also a Tiergarten (=animal garden) in Berlin but this is a giant public park in the heart of Berlin adjoining the Zoo on one side, while the Tierpark (= animal park) is located in whole different area
People often get these confused.
To give you a bit more background: The reasons why there are two zoos in Berlin and why both have such fantastic collections are historical. The Zoo Berlin with it's adjoining Aquarium (alone worth it for the tuataras!) is the oldest zoo in Germany founded in 1865 and is located in the Western part of the city. When Berlin was split in the four areas of the allies after WW2 the American, British and French sections formed the Western part of the city (which included the zoo) and the Sovjet section was the whole Eastern part of the city (which hadn't a zoo back then). Then we all now Germany and also Berlin got split up in two states. But before the wall was built, the authorities in East Berlin had a problem with their citizens visiting the zoo in the Western part. To prevent this, they started building their own zoo in East Berlin - the Tierpark which finally opened in 1955. From this date on in the whole era of the Cold War, Zoo and Tierpark Berlin basically had a proxy war. If one of the zoos got a special animal, the other one acquired an even better species to be better than the enemy in the other part of the city, so the collections in each zoos were growing and growing. That was very political back then and both Berlin zoos were basically the signature project of its state.
After the reunion of the two German states, the Tierpark in the East had a lot of financial trouble and was almost closed, also because the Zoo didn't really want to accept the Tierpark at first (they were afraid only one zoo can fit in the city) and in general the Tierpark was always a bit in the shade of the more well known and central located Zoo. Eventually, the Tierpark could be rescued because the right director managed to show the Tierpark is not a competitor but can complete the Zoo.
Today both zoos are managed by the same director who is working on giving both zoos more of its own signature. While the smaller Zoo is a classical old city zoo with a lot of historical building and top seller animals like giant pandas, the Tierpark at the moment is lacking quite a few top seller animals (no lions, seals, great apes, elephants or rhinos) but it has much more space (as
@Marvinb said) with large habitats for large groups of animals (especially a lot of rare ungulates like Somali wild ass
or three subspecies of takins ) and great landscape habitats like the new savannah (which opened last year) or the also quite new Himalayan area (opened in 2021/2022). At the moment, the old pachyderm building which used to show Asian and African elephants, Indian rhinos, manatees and earlier in the years even pygmy hippos and white rhinos is currently converted into one of the biggest elephant houses in Europe. So yeah, you definitely should visit both zoos.
Edit: Oops, this text went a little out of control and is much longer then I wanted it to be!
now I'm in Ely so very much dependant on the very unreliable trains. But cars are so expensive these days. 
a lot of good reasons to visit Berlin but that's mine!
