See my recommended Macs for photographers and similar higher-end users.
Today, Apple announced two worthwhile but expensive new options for the iMac Pro.
Up to 256GB memory (+$5200!) makes the iMac Pro much more “pro” and clearly differentiates it from the 2019 iMac 5K, at least for the tiny fraction of users who can use and afford it.
The improved GPU option is the +$700 Radeon Pro Vega 64X with 16GB of HBM2 memory. It looks to offer incremental gains over the Vega 64 option.
The iMac Pro comes at a premium, but it does have dual Thunderbolt 3 busses, a big plus. But my bet is that the 8-core iMac Pro will substantially underperform the 8-core 2019 iMac 5K.