What is the difference between calculus AB and BC (difficulty, workloads, etc.)?
Assuming that you can choose between AB and BC immediately after completing precalculus, the difference is that BC covers all of the AB material plus some additional material. BC covers what is ordinarily covered in a year of college single variable calculus, while AB covers just what is in the first semester or a little more. So BC will cover material at the same pace that college calculus courses will cover material, while AB moves at slower pace that is more like other high school math courses.
At some high schools, AB and BC are each a year long course, with BC covering only the remaining material after AB, so each course runs at a slower pace more like other high school math courses. At such a high school, it would take two years to cover what is ordinarily one year of single variable calculus in college.
https://secure-media.collegeboard.org/digitalServices/pdf/ap/ap-calculus-ab-and-bc-course-and-exam-description.pdf describes both AB and BC. In the concept outline section, topics which are only included in BC are indicated as such.
AP Calculus AB is equivalent to Calculus I. AP Calculus BC is equivalent to Calculus I & II. Calculus C is equivalent to Calculus II.