Green teas are made from leaves and buds. In general, they are much softer and lighter than black teas, but more intense and with a stronger body than white teas.
At present, green tea is produced all over the world, but some of the best are still produced in China and Japan. Chinese green teas are softer, sweeter, more aromatic and delicate than those from Japan. Chinese green teas are generally light with subtle notes of cooked vegetables and toasted nuts. Japanese teas are more intense and astringent, and in their sensorial experience notes of raw vegetables, seaweed and lemon are predominant.
Another important difference is in the production process. Fixing (the aim of fixing process is to deactivate the polyphenol oxidase enzyme and so stop oxidation) in China is traditionally carried out in pans set over wood stoves or coal kilns, in woks, hot metal panning machines, (or occasionally, when manufacturing sencha-style teas for teabags, in steam tunnels). In Japan steam is almost always used.
Each fixing method will give the produced tea a particular flavour. For example, teas fixed in woks or pans have sweeter notes and are more aromatic. This happens because the fixation is done more slowly and the teas have time to develop aromatic compounds. In this type of fixing (with a temperature higher than with vapour) glucosides are produced, which are a glucose derivate and produce a very agreeable toasted-sweet flavour.
In Japan the time of fixation is extremely short and steam is passed over the leaf.
Chinese or Japanese green tea, both can be excellent. You can't say that one is better than the other with objectivity, because each person has their own taste.