i did s1 for as last year and i found it easy, got a b but a lot of people got c's and much lower. For some reason they couldn't get their head around it, a friend of mine has got a in modules c1-c4 but got an e in s1 so is retaking it next week, however my cousin got e in both c1 and c2 but got an a in s1.
im doing m1 right now and i hate it, i need at least a b if i want to stay on track for my a but i would rather do s2, i heard from my teachers its much more interesting and easier, my cousin with the two e's is teaching it to herself and finds it easy, even my friend who got an e in s1 would now rather do s2 than m1 after all the initial learning, but its totally personal preference. all the people that do physics love it and would take it over s2 but everyone else doesn't. The majority of these people are retaking s1 due to bad results and after finishing the learning they have realized they would prefer to do s2. The teachers were going to give us a choice last year but they didn't. I remember one person asking their maths teachers why we don't do s2(this was whilst we were learning m1, right at the start) and she responded that most people failed s1 so how would we understand s2!
Woo that was a long reply sorry