I can help You as I visited both zoos November 2007.
The tiny Old zoo is located close to the central train station. It has a quite interesting collection of reptiles & amphibians (komodo dragon, Cuban iguana, Utila iguana, an Emys breeding program...), a small monkey house (with Siamangs, Red Ruffed Lemurs...) and 1,5

bird houses as well as a small museum. The old zoo really lives up to its name and is currently renovated.
The New Zoo is located a few miles away; You need to take the "Tram 6" or bus 50, but beware: it is poorly described and many people don't speak English. Just get a ticket for 20 minutes at the kiosk as there are no tickets sold in the vehicle. The New Zoo has an interesting Nocturnal house (Common Vampire bats, kowaris, Australian Water Rats...) and a Rodent house (with cururo, Eurasian hamster etc.), a butterfly exhibit (only opened in summer) and not-so-spectacular outdoor insect exhibit. It is embedded in a very park-like, quite beautiful scenery with large lakes. The zoo is gradually rebuilding, which means You have old-fashioned exhibits for carnivores (amur cat, European mink, jaguarundi, Palm Civet, White-nosed Coati ...), birds of prey (Steller's Eagle, Falkland caracara...) and pheasants as well as newer, roomier exhibits (Amur tiger, Grevy Zebra...soon: elephants). The ungulate collection is pretty interesting: Bawean deer, Philippine Spotted Deer, Thomson's gazelle, sitatunga, wood bison, Visayan warty pig...
Beside to claiming to have the greatest collection of cacti worldwide ( it was impressive, nevertheless)the botanical garden also offers some sub-optimal reptile & bird enclosures as well as a dark aquarium with pretty overcrowded tanks.
All in all, the trip is pretty cheap and if You need food & beverages or want to shop for souvenirs

, You can go to the large mall in the inner city.
Another advice: take the special train ticket Berlin-Poznan and back; all in all, it costs 40. Change the currency at the train station.