In short: yes, Intel is unfortunately the best choice right now. Not because they sell CPUs without an evil overlord, but because their evil overlord is the only one which we know how to (mostly?) disable.
Better the devil you know than the friend you don’t.
(warning: titanic wall of text) Gatekeeper explanation:
If you think of the ME or PSP, you might consider it as a secondary CPU - after all, it’s not the one which was advertised to you, you might not even know that it’s there and it is definitely not the chip with the highest performance. However, it is the most important processor in your system. The CPU has its own privilege levels, used to separate users from one another (enforced by the kernel), users from the OS (normal/root user); and OSes from each other (virtualisation). There are even parts of memory that the CPU cannot access - the system management mode (SMM) tables and also the parts where the ME/PSP stores its own code and data.
The ME has no such restrictions. It can read and write every single part of memory without interference - even the SMM region. It can peek and poke into every single hypervisor or kernel protected area. If your system has selective memory encryption, “trusted computing” functionality or other similar things - that’s the ME handling that. It takes over, does something and then returns control to the CPU without the CPU having any idea as to what has just happened. It even determines whether your system will even boot up - the ME shuts down your computer in 30 minutes if it’s not happy, AMD chips won’t even start until the PSP explicitly releases the reset line.
It will only execute code which carries Intel or AMD’s digital signature. You have absolutely no control over it. In short, with something like this active and running, you are not the true owner of your computer. This chip is. And so is anyone who can run code on it.
When I say “gatekeeper”, this is what I’m referring to - this tiny little “secondary” CPU is the real master of your system. It might be quiet for the vast majority of the time, but if it speaks up then nothing can disobey its orders.
All Intel CPUs after Core 2 have the Management Engine built in to the CPU itself. Core 2 had the ME as part of the chipset (I actually had an nVidia chipset for my ancient Core 2 system, so it didn’t have this management engine. It is, however, still vulnerable to Meltdown).
AMD’s equivalent, the PSP, got more attention recently when it was noticed that all Ryzen chips would have the PSP built in. Their older desktop CPUs, the FX series (sometimes called the “bulldozer” series, the particular one I’m using right now is an FX-8300), did not have this “feature”. This means that the master of the system is still the user who set it up. It should be noted that the PSP is not a new addition with Ryzen, it existed in their APU series of chips which were low power integrated graphics laptop parts
As for why they said “no” when people asked for the PSP’s code to be released, I imagine that the two main reasons are:
a) They probably licensed the base OS from someone else and therefore can’t give it away.
b) It’s what implements any DRM stuff which their chips support, which they would be explicitly prohibited from sharing.
c) The paranoid amongst you would give a third reason: they don’t want their backdoors being publicly known.
Honestly, the community made the wrong request there. A much better wish would be “please release a barebones firmware which does the absolute minimum required to boot the system and then halts the PSP, we don’t need any of the special sauce, just the signed image from you and also the code with a reproducible build process so that we can see that what we compile matches what we get”. That wouldn’t have the issue of releasing any proprietary code, it wouldn’t expose any DRM stuff and it would have been harder for them to reasonably say “no”.
Better for us, but less likely and requiring more effort on AMD’s part would be “we’d like a signed stub firmware that does everything above but can also load another firmware that we make and build ourselves”.
In any case, this is just wishful thinking on my part.