I took the PSPO a couple of years ago and am now looking at taking the PSM. How does it compare? Is it just a different a different 'chapter of the same book'? Harder/ easier? (I did ok on it; I studied on my own - I'm a terrible test taker, so throw a party when I pass with an 87%) And please don't get into the philosophical why, it's a story lol.
PSM-I is both the base assessment, and the Scrum Master assessment. Taking the exam, keep in mind "I am the Scrum Master". Questions are both general about Scrum, specific about the Scrum Master and about you as the Scrum Master.
For me, PSM-I was slightly (I'm guessing it was 1 question) better than PSPO-I. So I would say it's comparable in difficulty.