Hi C.,
My family and I rotate passes on a yearly basis between Lowry and MOSI and had maintained our Busch Gardens/Adventure Island passes from 2002 until this past summer. I guess it just depends on what you like to do and how much $$$ you are willing to spend. You can get a season pass for Adventure Island only during the summer, but since it's a seasonal park, you'd be out of an activity the rest of the year. If what you are looking for is the zoo only, I recommend Lowry Park because it has a larger variety of animals even though it's smaller in size. Also, little kids, like your son, usually enjoy the zoo more than Busch Gardens, because of the water features, the petting zoo, the small kangaroos, and the fact that is won't be a major production to walk it with a toddler. However, Busch Gardens is wonderful for walking, and there is certainly always something to do there, from a show, to dinner, etc... Some things to keep in mind, if you get a Lowry Park zoo pass, you can "swap" memberships every September with MOSI and the Florida Aquarium and see all three attractions using your membership. Also, your Busch Gardens pass only works for that park, whereas the Lowry pass can give you discounts and/or free entrances at other zoos around the country.