Ive seen and tried 3 of the main Korean peppers for kimchi/ferments. Two look almost the same but one is hotter and used for making pepper paste instead of powder for kimchi. The other one is much larger and milder. Similar to a milder Anaheim. They are usually green at the Korean market but occasionally they do have them ripe too. They are often used to make vinegar/soy pickled peppers and green chili kimchi. Its more of a "fresh" type kimchi.
The ones i grew last year were from CCN. They made a good powder but not quite what i like for kimchi. It worked better for seasoning soups and stews. Kitazawa has some gochu seeds i would like to try sometime. I think one is even an heirloom.