I’m interested in either taking AP Biology or AP Chemistry next year. Which one should I pick? I’m much more interested in chemistry, but I do want to have a wide range of subject classes and AP Bio is more popular at my school. Which class is more challenging?
Depends on the teacher.
About 51% score 3 or better on the AP Chem test vs. 64% on AP Bio, which should give you an indication of which is most difficult on the average. But as @Empireapple noted, a tough or bad teacher can skew an individual class.
Are you good at math/will you have completed Algebra 2 by the time you take this course? If not, Chem will likely be much harder than Bio, as it has a greater math component. As previous posters have stated, a bad teacher can also make one class much harder than the other.
Otherwise, just choose the subject that sounds more interesting to you. You’re only a sophomore, so you might be able to take both over the next two years.
As long as you have completed a sequence of bio, chem, and physics then you can take whichever course you are more interested in, whichever has the better teacher etc.
Do you know your intended college major? If it’s engineering, I would pick chem over bio.
- What do you want to major in? If engineering, take AP chem. If pre-med type thingss, then either. If not STEM, take the one with the better teacher.
What is the likelihood that you will have to take Chemistry in college? If it is high, I would take AP Chem. College Chemistry is one of the great weedout classes. Anything that you can do to avoid it or be better prepared will pay dividends in the future.
OP is a prospective physics major. I like @Eeyore’s advice 
but would go with whichever teacher is reckoned to be the best teacher (not ‘easiest’ or ‘most fun’ or whatever). Chem isn’t required for physics, but there is some overlap down the road, so having the basics can be helpful (my physics collegekid now gradschoolkid is (mildly) regretting not having done any chem in college)
AP Bio requires background in Chemistry. If you don’t have any previous Chemistry class, AP Bio is a strech.
Look at prospective colleges…at my alma mater, a BS in Physics requires Chemistry while a BA does not.