It might be placebo, but I find 3M better for upscaling, when I usually set CFG quite low and use a generic prompt that doesn't describe any localised element of the picture.
In my tests it's basically been 50/50, i probably did ~40 or so comparisons when i was testing samplers and i felt like there were a couple that seemed really good on the 3rd order one, but idfk, it was very very close, I don't know if I saw a single gen where one of the two was bad but the other wasn't.
Afaik the second order (2M) version is the recommended one to use for guided sampling vs the 3rd order one.
From here: https://huggingface.co/docs/diffusers/v0.23.1/en/api/schedul...
> It is recommended to set solver_order to 2 for guide sampling, and solver_order=3 for unconditional sampling.