What changes with thickness
Usucha at 2 grams to 60 to 70 milliliters is light bodied, foamed, and built to drink in one or two minutes. Koicha at 4 grams to 30 to 40 milliliters is unfoamed, glossy, and concentrated. Doubling the powder and halving the water makes umami and sweetness much more obvious, and any astringency much harder to hide.
When to choose koicha
Koicha is reserved for the highest tier matcha a producer offers, the kind labeled by Ippodo or Marukyu Koyamaen as suitable for the thick preparation. The best koicha tastes of seaweed, sweet cream, and stone, with no scratch on the back of the tongue. A mid tier matcha pushed into koicha thickness will simply taste bitter.
When to choose usucha
Usucha is the everyday preparation. It works with a wider range of grades, scales easily to lattes and iced drinks, and forgives small temperature and ratio errors. If you are learning a new tin, start with usucha so you can read the powder before you commit to the more demanding thick preparation.
Reading mood and occasion
Koicha is slow and shared in small sips, often at the start of a longer tea sitting. Usucha is brisk and personal, finished from your own bowl. Many drinkers keep one tin for koicha use only and a second drinking grade tin for daily usucha, which also helps each tin stay within its 60 to 90 day freshness window.