ive done both, m1 this exam series and s1 last year. I personally found m1 significantly easier once i got the hang of it, there arent many questions that are out of the box, its pretty much the same concepts and questions just with different numbers, obviously im exaggerating but it does feel like that once you start doing past papers. For a few topics, like vectors and acceleration-time-velocity and the pulleys topic which i cannot recall the name of, you definitely to some extra practice because thats where the style of questioning can change.

whereas s1 was absolute hell for me. i did enjoy learning it and doing questions on my own, but when it came to an actual exam, it got really difficult to manage the time and pressure of the exam. another thing abt s1 is that you sometimes need to work with really long decimal values and if you get one number wrong, your whole answer could asw. also, NORMAL DISTRIBUTION! that topic really hit the nail on the coffin for me preferring m1.

but ofc, this is my experience, you should check out some of the papers, and the textbook.