To be clear, I am assuming the same hardware budget for both the on-disk and in-memory case. Ceteris paribus and all that. An in-memory model has no inherent advantages in that case because the same memory is potentially available to both for a given workload. Another way of stating it is that SSD won't slow you down.

Ironically, in practice relative performance between these two models is all over the map. Quality of implementation has much bigger impact than the abstract model. I've seen high-quality disk-backed database engines wipe the floor with in-memory implementations on the same hardware and vice versa.