Sorry about the delay. Just got alerted by @CookiePuss to this thread, even after tagging me in it days ago. Sorry about that. Hopefully the in depth explanation and discussion to nerd over makes up for it :P.
If you haven't seen it already, I would recommend looking here to see the work I did for performance testing Ryzen CPU's in action in APB @ 4K in both the current engine and the upcoming Engine Upgrade. Spoiler: they work quite well.
The graph listed is the same graph provided in the thread linked above but just a bit higher clarity.
Now with this graph, I can say personally that either AMD or Intel for your CPU choices would be equally valid, depending on if you're using the system just for gaming, or for more. There are anomalies that both Intel and AMD suffer from, which I will go into once appropriate. Before all of that, I have to go through the current situation.
The current situation
As of right now, APB is a tightrope. A lot of the problems that players face today are from how tight some of the constraints are, and how the main game thread is tied up as soon as something happens, and with little relief until the task executed is complete.
The biggest problem is the Garbage Collector, which is one big double-edged sword. On the one hand, it allows the game to run. On the other, it can make the game run extremely poorly. It's necessary for the 32-bit memory constraint to work, but at the same time strangles the main game thread every sixty seconds as it does a fly-by. Unless you are willing to edit the GC settings in the files (don't, it's very easy to get it wrong), you will suffer with everyone else on the matter and soldier on. This is something that all platforms suffer from, you're not getting around it.
But more than likely an even bigger core problem to APB is that it does not support multi-core. It can use more than one thread, however it cannot scale past two main threads, used by the game executable itself for the main game thread and the render thread. This does not include child processes like Vivox Voice Service and the APB Exception Handler (APB_Catcher). APB historically prefers clock over cores, but can benefit from running on unique cores. This, again, is something that all platforms suffer from, you're not getting around it.
The two issues I've listed above would be rectified with the Engine Upgrade, that much can be said. So you're going to need to look ahead...
The look ahead
In the future - APB will largely be a much more "fair game" situation, where the user will be able to not experience drops every time they look at LeBoyce's awesome afro. Oh and also not crash every time the memory map is exceeded. So your choice should be influenced by that.
With that in mind - looking ahead to your potential CPU choice would be moving to a new platform regardless of what you do. Unfortunately that is the way of the world since forever for Intel, and a move to AMD would require it anyway for AM4. Let me outline very quickly some bullet points on both CPU manufacturers (i.e benefits and drawbacks, with APB specific items marked) Intel
(For APB) It is currently seen as the best CPU for the current engine, and it can push high framerate.
Up until recently was still superior to AMD for gaming in general
More established second hand market
You are required to purchase a new chipset & motherboard if you want to upgrade
Prices are very high compared to AMD offerings
14nm++++++. 10nm has been so slow to market, that 14nm was recycled again and again. Aged well, but it's not worth it.
Expand-ability via PCIe (for NVMe SSD's etc.) is limited on the budget side drastically. You need to fork out hundreds for more PCIe lanes.
You are not required to purchase a new chipset & motherboard if you want to upgrade.
Ryzen 3000 is essentially holding parity to Intel for single core performance
Multi-core Ryzen is vastly superior to Intel multi-core.
Much cheaper CPU's for much more cores and now similar performance
(For APB) Can have inferior performance by a few percentage points (Ryzen 1st Gen)
May not have an organic second hand market yet (it's getting there though)
Overclocking is essentially not possible past a certain point. Mild overclocks are possible but more substantial offsets are limited to LN2 cooling.
First Gen X370/B350/A320 motherboards can be junk in terms of quality. 3rd/2nd gen boards are A-OK.
As you are going to a new platform in any regard, I would personally recommend you get a Ryzen 3rd Gen CPU (3600, 3800X or the 3900X depending on your budget), simply because for the price, you will be getting into a platform that can be upgraded past one single generation which would be very important if you just want to upgrade a CPU down the line for instance. Intel for the price really isn't competitive and even though the 9900K can be competitive in gaming, if you do content creation (I assume you do since you got a Mac), the performance of the AMD CPU's in that space is second to none. Again, it is down to what workloads you do.
Hopefully that helps a little bit. If you need me to answer any questions past that just mention me and let me know specifically what you want answered. And that goes out to anyone in this thread.