There's another possibility : none of them has better health benefits than the other. Or, saying it differently; both of them are good for health as long as they are associated with a healthy way of living (food, physical activity, no smoking, limited alcohol, etc...).
No less, no more.
If tea (or anything else) was the answer to all health problems, it would be world wide known!
That said, tea is rather a healthy beverage and rather than japanese vs chinese, you should try to find serious vendors selling well preserved teas that were grown with no or low pesticides/fertilizers.
There are japanese tea producers using a lot of chemicals, chinese farmers that grow organically and vice-versa. I don't think you can make a clear statement on whether one is "better" than the other.
For sellers, I think that http://www.thes-du-japon.com/
and Ippodo are highly thought around here.
And remember that there's so much more in tea than the real or alleged health benefits!