Generally, dark brown gets more use than tan, in my book. Although they are both essential.
The only caveat is that White's dark brown is REALLY dark brown, like almost black.
A brown CXL that has more mid tones would make a good third option.
B-ewing would you be in the market for a $1000 pair of White's boots?
If you are, go for it, get the horse butt. HB is pretty much the king of dress leathers (you know better than I) and should be well worth it (if you have 1k to spend). I’m glad I’m getting my navy CXL SDs, but someday I really want to get them in cordovan.
SD shoes in cordovan are available hear:
Update: This is really interesting...
"The strip and the shells are pure vegetable pit tannages, and the horsefront can be tanned either with or without chrome – note: horsefront a.k.a. front quarter horsehide (FQHH), double horsefront (DHF), and single horsefront (SHF). The strip (a.k.a. bark strip, russet horsehide strip, natural horsebutt strip, north of cordovan, and others) is thick relative to the front, and typically ranges from 4 – 10 oz, or 1.6 mm to 4 mm. This is great leather, though it’s a little funky to work with due to the shape(Horween)."
"The shell has no grain, the strip in the center has very pronounced and coarse grain, and the two steerhide examples have very fine, but still very pronounced grain. These are all full grain products (Horween)."