I am a rising senior who has taken both AP Biology and AP Chemistry.
Like others have said, if you have already taken pre-requisites (if your school has any), then I would take the AP course that you can take. If you have the option to take any one of them, I would first look at interest.
For my school, I had the option to take both, and so I did. But, I had very different experiences in both classes. My AP Chem teacher made the class much more easier than other AP Chem teachers, and so while the class was somewhat easy, I had to do a LOT of self-studying before the AP Exam. My AP Bio teacher made the class slightly more rigorous, so there was less self-studying involved.
Regarding the curriculum of each class, AP Chemistry I feel is a significantly harder class. There are concepts towards the end of the year, that if you have not taken a Chemistry class before, are very hard to grasp. Both touch on important bio concepts, like reactions, organic compounds, etc. However, AP Biology does use DNA studies and Genetics as a significant part of its curriculum while AP Chemistry does not.