Chester is lucky it gets a mammal category, otherwise it would almost certainly have been exit....
Both:
Asian elephant , Lowland tapir, Grevy zebra, Onager, Eastern black rhino, Balabac chevrotain, Southern pudu, Javan banteng, Okapi, Red river hog, Western sitatunga, Red duiker
Chester keeps:
Malayan tapir
Greater one-horned rhinoceros
Babirussa, Common warthog, Negros warty pig
Barking deer, Eld's deer, Visayan spotted deer
Lowland anoa, Red buffalo, Roan antelope, Kirk's dikdik, Eastern bongo
Rotschildt's giraffe
Cologne keeps
Przewalski wild horse
Common hippo
Bactrian deer
Blackbuck, Common impala, Persian gazelle, Greenland musk ox, American bison
Reticulated giraffe
(Nile lechwe will come to Cologne this year)
Overall Chester has more species but Cologne has a higher diversity, as they do keep Common hippo....