Looking at the above, it looks to me like it’s a question of if you primarily want to study plants in and of themselves, or if you’re more interested in how plants are used in industries such as agriculture. From the sound of it, plant biology is the former and plant science is the latter.
Given that you specifically mentioned botany, I’m guessing plant biology is closer to your interests. Take a look at the descriptions and programs yourself and judge though. Here are the course catalog pages for each major:
Plant biology: http://catalog.ucdavis.edu/programs/PLB/PLBreqt.html
Plant science: http://catalog.ucdavis.edu/programs/PLS/PLSreqt.html