The IB program is harder than AP program because students need to put in more time and efforts when they are taking IB program. Apart from the advanced topics covered in IB class, the exams too are hard and students have to cover all the topics within a short period of time and it even gets more complicated during the exams because IB exams are always done in May, just a month before the end of the school year.
In IB classes, an internal assessment are done for each class. Few examples are an essay for Math, a classic literature for English, a student design lab for Chemistry, an independent research essay of up to 4,000 words etc. Students must also do 150+ hours of volunteering work and also take the Theory of Knowledge (TOK) course. So because the exams are in May, the classes are faster with a higher workload than AP program. Taking all of this still doesn't guaranty you an IB diploma. You have to pass with with a minimum passing score of 24 points and at least a satisfactory on the extended essay.
Both IB and AP are great programs, and the good thing is universities accept the both of them but IB is more difficult than AP.