In terms of a 35mm lens don't expect to get much of anything good in terms of photos that's outside of either zoo's reptile houses. Fort Worth obviously has a better reptile house, but both have stellar collections and plenty of opportunity for photography (Iv'e spent hours at a time in both reptile houses for pictures of the collection). Fort Worth's MOLA is also better for photography though due to the fact they use real plants in the enclosures and they use some kind of light which brings out contrast.
As far as birds go Fort Worth has a very overlooked but fantastic waterfowl collection, with species such as smew, eurasian wigeon, tufted duck, falcated teal, white headed duck, and Indian spot billed duck (those are just a few examples). Dallas itself has an impressive waterfowl collection but not to the extent of Fort Worth. Both zoos have great raptor collections as mentioned, both with Harpy eagles, palm nut vultures, and white backed vultures. Fort Worth also African crested eagles and Swainson's hawk, while Dallas having more vultures such as lappet faced, hooded, and ruppel's.
Giants of the Savannah at Dallas is spectacular, but Fort Worth has it's own new Savannah which is not quite as good but very nice. Fort Worth's new hippo exhibit, I might add, is much better than that of Dallas.
If you haven't already you may want to flip through both zoo galleries to see what species they have you might want to see.
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@Hyak_II don't forget about Fort Worth's red wolves, swift foxes, pallid bats, lesser long nosed bats, and kangaroo rats, I'd say those are pretty unique too, especially since you can't find some of them (such as the bats) in a zoo anywhere else in the world.
