Which one is commonly used to express the choice among the same kind, for example: which one do you buy (among several different brands of mobile phones)? Which mobile phone will you buy? So which clothing store is the best? Which clothing store is the best (among several clothing stores)
What is Chinese is "What?" Choose between different categories, such as: What do you buy? Use a pen, a knife, an eraser, a ruler, etc. ) What will you buy?