Actually, the 50mm deep carbon rim is the same price as the 38mm, so
I could build a finished wheel at the same cost.

But looking at the instruction manual for Tni's carbon rims,
the upper limit for spoke tension is 130kg for the 38mm case, while
for the 50mm it's somehow only 100kg.
You could argue that rim deflection is less on the 50mm (stiffer rim) so
it balances out, but
I'd recommend the 38mm, which I can build really tight.
130kg for the 38mm is actually a pretty high tension you can achieve.
Depending on the manufacturer, aluminum WO rims typically max out around 120kg.
There are a few things to watch out for, but the 38mm builds up just like an aluminum rim.
The reason I thought about using Tni's carbon 38mm as the material for Nomu Lab Wheel No. 2 is that
when buying parts separately and building it myself, it comes out cheaper than Tni's factory models, and
it's a rim that can handle tension up to 130kg.