I've heard that getting a Chinese (multientry) visa is easier if you've had one before. Hence, I would use the Canadian passport unless the USA one has all the previous visas; then I'd reconsider. Chinese visas can be done fast (assuming there's no problem) for extra fees. Also, USA consulates for China don't accept visa applications that are mailed or FedEx'd, which means that if you can't deliver and retrieve the documents in person, you must use a visa service. I don't know whether this is different in Canada or if you can use a passport or apply easily through a country other than your residence.

Are you sure that China is currently issuing the special permits for Tibet? I had heard recently that the Chinese had closed Tibet to foreigners again, but I don't remember the source or any details.