I perfer fall over summer because there's no annoying bugs in the way, the heat is not burning my skin, I feel cooler in the fall, another main reason because I was born in the fall.
Depends, I can scavenge a lot of cool stuff in fall more easily, but herping (herpetology, looking for reptiles and amphibians) is a lot more easily done in the summer. My personal preference is autumn, though!