Sources are conflicting, but the general consensus I've seen is that the Intel ME has direct access to your networking hardware, while the AMD PSP does not, and basically just exposes interfaces to the CPU.

If accurate, this is a significant functional difference as far as I'm concerned.

But, what's the old saying again? "The only truly secure system is one that is powered off, cast in a block of concrete and sealed in a lead-lined room with armed guards - and even then I have my doubts."