OK, here I go for my first post...
I'm buying the 27" decked out, except for the RAM, and install that myself.
I'm debating the 1TB/3TB option, as if I need more HD space, I can go the Thunderbolt option. I figure 10Gbps > 600 Mbps (SATA) when I need it, but I do want the Fusion drive for the internal, and debating SSD externals with the Thunderbolt.
Since I'm using the word, Thunderbolt so much, and previously (in my PC days), the external interfaces were the bottleneck, and also with it being 10Gbps vs. SATA 600 Mbps, would the HD be the bottleneck in this setup? If so, then internal hard drives are no longer the limiting factor that they would appear to be, and the next issue is cost of the external drive enclosure(s).
There is no advantage of Thunderbolt over USB-3 when using hard disks as both interfaces are limited by the speed of the disks. If you are using external SSDs then you probably will want to use Thunderbolt.
Here are some results of tests I did recently:
https://forums.macrumors.com/posts/16377370/
-howard