Well strangely enough, I was persuaded to vote for RZSS in this poll, I'll give my reasons below.
I know Madagascar was included in the voting criteria of this poll but personally I never really consider / define Malagasy fauna to be "African" but rather as Madagascan. So basically I just write off the species kept at Marwell from that region.
That left Marwell basically with their impressive collection of African hoofstock, cheetah, yellow mongoose, cusimanse and a couple of reptile and bird species. But as you point out Edinburgh has an impressive array of African primates and I am someone involved in primate conservation so there is admittedly a bit of a bias there.
It is something of a full house at the zoo with both great apes and monkeys : Chimpanzee, Diana monkey, gelada baboon, drill, L' Hoest's monkey.
Other considerations that made me vote for RZSS were other African species represented that I felt represented the fauna of the continent more broadly then Marwell's focus on hoofstock.
African hoofstock: Nyala, red river hog, Kirk's dik dik, Grevy's zebra and the pygmy goat (sure it is a domestic but I included it anyway).
Other African mammals : Cape porcupine and meerkat.
African reptiles kept include: Pancake tortoise,
African birds kept include: Vulturine guinea fowl, Egyptian vulture, Eastern white pelican, East African crowned crane, black stork and Waldrapp's ibis.