For AMD card, it’s either OOTB or not supported at all. For Pre Maxwell card, one of the big advantages of the Nvidia card is that OSX has native support for those card. So, you have the web driver as back up. If Apple suddenly stop supporting your card, or it has bugs etc, you can switch to web driver and still can be a happy user.
But since Maxwell, Apple stop supporting any new Nvidia card. Now, the Nvidia card’s situation is not much better than AMD card. If Nvidia stop supporting them, or provide buggy driver, users has no backup plan. Even worse, it can be a nightmare to deal with the web driver (black screen, boot loop, etc). It’s hard to know what Apple will do next to stop us using the new Nvidia card. e.g. a user now report even with the flashed card, a PRAM reset can still get into boot loop.
For AMD card, there is one more extra limitation, any non reference may not work in MacOS. For Nvidia card, AFAIK, the web driver can power any card as long as it’s in the family that is supoorted, regardless if that’s a reference design or not. But in AMD, clearly not the case. e.g. None of the single slot RX460 works in MacOS..
IMO, of the user need CUDA, then no choice, Nvidia is the way to go (however, MacOS most likely is not a must).
If the user only need mid level card, then AMD new card is the way to go (e.g. RX580). Just pick the correct card that should have proper driver support (study which GPU inside the iMac, MacBook Pro, or eGPU developer kit), then the life should be very easy.
If the user only need low end card. I still recommend the old 7950 / 680, or anything below that but can be flashed. Regardless their performance, those card still has best support, most mature driver, and able to provide boot screen can be a big advantage in some situation (e.g. in High Sierra with APFS, using boot screen is the only way to let the user choose between Windows / MacOS on next boot. Bootchamp can let users switch between OS, but not really let us “choose”).