Both are equally natural to hear. The only time I would favour one over the other is if maybe "September" is part of the next year. So let's say this conversation was happening in November 2015, but the trip to China is in September 2016, I'd probably say "in September" because September doesn't feel like part of "this" year... BUT either one sounds perfectly natural to me.

How about B?

B, I think. :)