Have been to Beauval, but never Frankfurt.
It is pretty difficult to vote against the French collection here due to how much more they have on offer, at least in terms of mammals. However, I simply cannot go for a 4-1, as I am not all too fond of a lot of Beauval's enclosures in this category.
For instance, the Puma and Persian Leopard (if the latter count) cages are probably the worst that I have seen for cats of their size, with limited climbing, privacy and space. I have never quite shared the same fondness of Beauval's bear and wolf enclosures as some other seem to; yes there is a lot of space on offer, but the enclosures are too sparsely and inappropriately planted to really represent a Eurasian woodland and offer the inhabitants sufficient shade and privacy. On my visit a few years ago, the heat was excruciating and the bears had very little shade to cool off. The Mishmi Takin enclosure is huge and offers some decent height, but I feel it could maybe do with a little more steepness - I confess I am nitpicking somewhat, though. Love the Snow Leopard and Giant Panda enclosures, as well as the raccoon and macaque enclosures to a lesser extent. When it comes to birds, I don't really like the pinioned cranes mixed with the takins, and there are quite a few other flight-restricted birds throughout the zoo, some of which I assume are category-relevant, but there is a very nice rarity in the Andean Cock-of-the-rock, free-flight in a large greenhouse.
I still feel as though Beauval's nice enclosures, rarities and larger collection give them the edge here, but there are too many substandard enclosures for said edge to be of a great margin, and this is coming from someone who (unlike a lot of ZC members, it seems) is really fond of the place. 3-2 for me. Perhaps someone more familiar with Frankfurt can detail their non-mammal offerings in this category and I could be tempted to switch.
Below are some photos from Beauval to demonstrate my point about the inconsistent exhibitry.
@Maguari - Puma enclosure (no photos of the leopard enclosure in the gallery, but from memory it is very similar)
@lintworm - Brown Bear (and Arctic Wolf? they weren't mixed on my 2019 visit but this may have changed since) enclosure
@twilighter - Mishmi Takin and crane (unsure of species) encosure
@twilighter - one of the Giant Panda enclosures
@twilighter - Snow Leopard enclosure
Another factor in Beauval's favour is the theming of the 'Hills of China' enclosure, which in my opinion is very nicely done. The mist machines and panda topiaries as you approach made my first ever encounter with the species even more memorable, the pagodas are tastefully done and the many tea leaves and bamboo trees that surround it do some botanical theming.
So all in all, I do feel as though Beauval deserves to win, but they are perhaps not as strong in this category as one might initially think, and could be open to persuasion.