They are frequently confused because both live in every MSP and both create tickets. The distinction that matters is which question each one answers. RMM answers "what is happening on that machine". PSA answers "was that work profitable, against which agreement, and who did it".
Most MSPs run both and connect them, so that an RMM alert opens a PSA ticket and the time spent lands against the right contract. That integration is where the value is, and where it commonly breaks: when the link is loose, work gets done without being recorded, and the reports quietly stop describing reality.
If you are choosing where to invest attention first, note that RMM without PSA leaves you efficient and unable to tell whether you are profitable, while PSA without RMM leaves you well-informed about work you are doing manually. Neither is a substitute for the other, and a tool advertised as both is worth checking closely against what your team actually needs each side to do.