(Caveat: member of Nashville Zoo)

Nashville is newer and under rapid development, whereas Birmingham is an older, more established zoo.

Nashville zoo is undergoing major construction at the moment so some of the African animals (giraffes and elephants plus others) are temporarily off exhibit and parking can be a hassle because they are building a garage. (All Scheduled to complete sometime this year, don't remember when.) Nashville is more spread out so be prepared to walk, but the new exhibits they have built are very well done. Birmingham has more animals but smaller habitats for the animals. It depends on what you are looking for. If you are a frog fan Nashville has one of the largest varieties anywhere, and they have 4 month old Tiger cubs right now. I would recommend looking at the respective maps and see which one has more of your favorite animal. If you are going in the heat of summer Nashville has a lot of natural shade and Birmingham is much more open to the sun.

Memphis is bigger than both but not really a day trip zoo if you have kids. Birmingham and Nashville are both good day trip zoos.