I think that Bok Choy grows faster than tatsoi, but tatsoi can tolerate much more cold. Sow it 4-6 weeks before the last frost and successively after that. If summer catches them before they're full-sized, just harvest the baby leaves as delicacies.
One source claimed their variety of tatsoi was cold hardy down to 15ºF, but I never tried that test! Seventeen degrees below freezing!
I think that "Takuchoy" is Mandarin for "Tatsoi".
The only unusual varieties of tatsoi I've seen advertised were a red F1 tatsoi from Kitazawa (#446) and a Red Violet F1 Tatsoi F1 from Nichols (VAG595). It's as if tatsoi is considered fairly generic, compared to green stem Bok Choy and Chinese cabbage, where everyone competes with slightly different varieties and hybrids.
Kitazawa also has a red-violet F1 variety they say is a Tatsoi / Bok Choy hybrid.
