Both contenders in this one find themselves in a pretty strong position, with two triumphs and no losses apiece..... but one of them is going to have this streak broken, with the coming debate focusing on the topic of TROPICAL FOREST. Judge the matter how you will - exhibit quality, species held, conservation efforts - and then make your cases here!
Voted Paignton due to their many strengths in this category: species large and small that are native and endemic to tropical rainforest / forests, species from across biogeographic regions that comprise the tropics, species across all classes whether birds, reptile, amphibian and mammals. Dublin / Fota by contrast seem to me to be an uninspired collection that focuses on larger ABC mammals many of which are held by zoos the world over. The conservation output of Paignton is truly enormous when you consider the Whitley conservation trust and the way it supports and funds conservation initiatives / projects the world over year after year. This is particularly the case with funding in-situ conservation within tropical rainforest biodiversity hotspots. Dublin / Fota by contast do not have that large a conservation output though they do financially support in-situ projects with rainforest species in Africa and the Neotropics like the golden lion tamarin, lowland tapir, okapi, bongo, orangutang, gorilla, chimpanzee, siamang.
I went to Fota for the first time last year and was very impressed. The primate islands are some of the best I've seen, and certainly challenge the view that cages offer more climbing opportunity. The downside of some islands is the indoor accomodation which ranges from decent to tiny. The Tigers and Asiatic lions also have great enclosures.