I’ve taken Calculus AB.
And I have one month to self study for either of the Physics.
Physics B doesn’t exist anymore- there is Physics 1 and 2, which are algebra based, but two separate exams (there is no calculus involved). Physics C is split into two separate exams, E & M and Mechanics, and is significantly harder because it is calc based. In fact, they are considered among the hardest AP exams. You won’t be able to self study two physics exams in a month- let alone just one physics C exam.
I agree with the above.
I don’t think there is anything “official” about which is harder; there are only opinions.
IMO, having taken a number of AP exams, including Physics B and both Physics C tests, E&M is by far the hardest exam and class.
I tried self-studying for AP Physics C E&M last year, and believe me, it was really painful. I procrastinated too, started ~three weeks before the test. The topics are really hard to grasp, such as electric/magnetic field, circuits, etc. I barely passed the test (got a 3).
However, for AP Physics C Mechanics, it is doable if you already took regular/honors physics and you are good at calculus.
For self-studying, I would suggest flippingphysics and MIT opencourseware websites. Both of them have really informative videos.
Good luck!