Hi Adam,
I have a similar observation. As you pointed out, the PCG solver is fast for transient flow models, because the matrix assembly is highly parallelized. According to my observations, the SAMG is quite fast for steady-state or quasi steady flow models.
Yes, the number of nodes are very interesting with regard to serial VS. parallel. Moreover, the number of cores are also interesting. More cores do not necessarily imply faster computations. Most likely a drastic / exaggerated example: if the number of cores are "high" and if the number of nodes are "low", a reduction of cores could even accelerate the computations.