China visa en route
 

We're leaving US for London/Paris/Saigon/HK/Shanghai/Beijing back to US in Oct-Nov.

Rather than get China visa in US ahead of time, what are the advantages/disadvatages of getting it in HK on the way?

I don't think there's any real advantage. For just 2 months, the visa will be good when you get there. The only visas I've waited to get, en route in another country, were times I'd be traveling longer and needed to wait, for them to still be valid on arrival. I've gotten a Chinese visa in India and a Burma visa in Hanoi for that reason only. When possible, it will save you taking time out from travels to get it before you leave home.

I should also mention, both instances I mention took considerable time out of 2 days each. It wasn't a problem for me as I had plenty of time. Doing it from the comfort of home, especially if through an expeditor, has the advantage of taking no time from your travels and why I urge you to do as much as possible before you travel. It appears you'll be moving regularly and may not want to wait as long as a week to get your passport back. It may be less than that in HKG but it's still time spent doing what could have been done in advance.

If there's no Chinese consulate near where you live (making it a hassle to obtain the visa beforehand), then it can be an advantage IF AND ONLY IF you have enough time in HK to get it while there.

These are the people I used for my last China visa:

http://www.travisa.com

It isn't expensive, they're wonderfully efficient and no need to be anywhere near a China consulate. Get it done in advance, on your own if it's convenient as rkkwan says, or use an expeditor like Travisa.