Because the PSM is ultimately responsible for the production, I’d say the role of PSM is at least slightly harder than ASM. The ASM may seem to work harder because they tend to be seen on their feet more, but the PSM is often doing the less visible, but equally hard work of wrangling the cast and creative team. That doesn't mean being an ASM is easy or stress free. Particularly on large scale shows, where both the PSM & ASM are professionals, the PSM may delegate a huge amount to the ASM.
I feel the hardest part of ASMing is sometimes what it teaches me about myself. While I love learning how other stage managers work (which is hard to do when you’re long out of school and usually the lone SM on a show), it can be a challenge to adapt to their style when it’s different from my own. I have to really focus on staying positive and keeping my perspective if their way of doing things frustrates me. I have to use good judgement about when to offer up alternatives, and when to do it their way because it’s their show. I have to support the PSM the way they need to be supported, which is not necessarily how I would want to be supported if our roles were reversed. In some ways the ASM/PSM collaboration is very similar, and just as delicate, as the PSM/director collaboration.
To be a good leader, I think you need to be a good follower first, so to me the PA-ASM-PSM ladder makes good sense. As a PSM, I like knowing that I’m not asking someone on my team to do anything I haven’t/couldn’t/wouldn’t do myself. I feel that a PSM’s skill set should include the abilities of PA or ASM types, but not all PAs or ASMs have the ability to PSM.