They are the same thing, choy sum in Mandarin and yow choy in Cantonese. Yu choy translates to oil vegetable and is used to produce oil. Choy sum/yu choy has slender stems that are pale green and firm with flat and lightly serrated green leaves. It is relatively mild and tender, with a slightly sweet flavour. Great in salads. Do not confuse it with Gai Lan or Chinese broccoli, which is larger, less tender and may have some yellow flowers. It has a more pronounced bitter and/or mustard-like flavour than choy sum. Best to cook with.