I've seen no solid evidence comparing Japanese to Chinese teas in terms of health. Unless it's studied scientifically, anything said on the topic is pure speculation.
Some of the reasoning I'm hearing in this thread also sounds slightly suspect to me.
For example, the steaming process does make the leaves broken, and this can make them infuse more quickly. But it also can make the tea break down more quickly in storage. Whole-leaf tea tends to retain beneficial chemicals like Vitamin C longer. Also, more of the chemicals extracted from the tea does not mean it is healthier. Antioxidants are something that can be beneficial in moderation, but in too high quantities, they can inhibit various biological processes. The antioxidants in tea, with the exception of the small amount of Vitamin C in green tea, are not essential nutrients, and are not as beneficial (and possibly not as safe in large quantities) as antioxidant vitamins like Vitamins A, C, and E.
So, I think there are theoretical justifications for it going either way.
I say, just drink what you enjoy the most.