If building and you are reasonably certain you know what stem length you need, and at what angle, then 1" can make sense.

If any doubt then 1-1/8" is a lot easier to swap to different stems, no need to remove the bar tape, shifters, cables, etc.... as needed on a threaded, just unbolt the threadless, swap stems and tighten bolts.

As well, I find threadless easier to install and to get tensioned correctly and am glad Soma opted to use 1-1/8 on their (and my Soma) frames instead of 1" as Rivendell does, even though both make nice steel frames.