G'day, personally would take the GSA as PRC has quite a few officially authorised BMW dealerships across the country if needed..The Yamaha XT660Z is not very common around PRC "People Replic of Changes".... Well - believe it or not but Mainland China has plenty of roads / highways in various conditions.....

Might become a major hassle and miles of "Red Tape" to enter PRC "People's Republic of Changes" in the first place and continue onwards to HK from PRC ~ just sayin' as driving / riding through Mainland China fully legally with overseas motor vehicles is quite costly and very bureaucratic. Many Chinese cities (municipalities) ban motorcycles within city limits, Chengdu (Sichuan) just recently joined the extensive list and outlawed motorbikes around the various city districts... Safe travels and my personal China rules below, came up with them decades ago and even more spot on nowadays....

TIC = This is China!
1. Everything is possible in China.
2. Nothing is easy.
3. Patience is the essence of success.
4. The answer "yes" is not necessarily an indication of agreement or confirmation.
5. "You don't understand our country" means they disagree.
6. "Provisional regulations" mean they can change the rules any time they want, even retroactively.
7. "Basically, no problem" means a BIG problem.
8. "Signing a contract" means the beginning of the real negotiation.
9. When you are feeling optimistic, think about rule number two.
10. When you are feeling discouraged, think about rule number one.
11. It’s easier to ask for forgiveness than it is to get permission.
12. If they tell you, that you know China very well, say NO - otherwise they'll show you, that you don't!

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Somewhere down the road in China since '89 ~ along the route I've learned the hard way that pleasing everyone is impossible, but pissing everybody off is a piece of cake....TBR