I’ve seen a lot of people talking about their Mac Pro choices who seem to be underinformed on how these CPUs work. (Intel really isn’t helping.) Here’s what I wrote last month, explaining how Turbo Boost makes each CPU (except the 12-core) much more similar than you’d expect until you get into workloads that max out all CPU cores.
In short: despite their advertised clock-speed differences, single-threaded performance in practice is effectively identical between the 4-, 6-, and 8-core models. There’s no “penalty” for going from the “3.7 GHz” 4-core to the “3.5 GHz” 6-core, for instance.
Now that the pricing is confirmed, I think the best bang-for-the-buck option is the stock $3,999 configuration (6-core, 16 GB, D500) plus whatever amount of SSD storage you need. That said, I’m going 8-core and D700 on mine: D700 because it’s not that much more, relatively speaking, and can probably never be upgraded (at least for a reasonable price), and 8-core because I do a lot of parallel work but can’t afford the non-parallel penalties of the 12-core.