If that's a new software / function, then It's really hard to tell.
In general, for computer / rendering, AMD card work quite well, and has better cost to performance ratio then Nvidia card.
However, it's very limiting to OOTB reference card, any other card may not work, also, unless it's an official supported card (e.g. HD7950 / RX580), the driver can disappear on any OS update. (R9 280X is also safe due to it has the same device ID as the D700, however, for unknown reason, only 3GB card works, 6GB card doesn't work at all)
If you don't mind to deal with Nvidia web driver (no display until web driver properly install and activated after every single OS update), then Maxwell card seems also a reasonable choice. The report about Pascal is not consistent so far. But Maxwell driver is pretty mature, and it perform quite well on compute as well.
IMO, if in doubt, Sapphire PULSE RX580 8GB is the safest bet, it is the actual card that shipped with the Apple eGPU developer kit.
Testing shows Vega 56 also work at this moment, however, it's right at the edge of the power envelope that the mini 6pin can deliver (it's 100% guarantee will draw more than the 75W official limit, but may just able to stay within the 120W real world limit in some operation). If C4D is really "heavy" for GPU, it can shut down the Mac it a second. So, without proper testing. Almost impossible to tell if this one can be a good card in C4D R19.
On the other hand, RX580 is very safe, it's been tested OK to run Furmark, that means nothing on real world should able to cause any power draw issue.