Speaking of China Post, it has an online tracking, except that it's not updated on a timely basis (hence, accuracy ??) for Registered Mail & EMS via its system, with specific tabs/links for items destined for USA (see link)
http://track-chinapost.com/
A cautionary note posted at the bottom is a big WARNING flag for the next 6 to 8 weeks (actually, it will probably extend into next February with the Lunar New Year)
Due to the Christmas Rush, China Post Airmail Parcels in this season are generally delayed by 1 to 3 weeks compared with regular season.
I had a small package mailed with tracking # (RR sequence) from Guangzhou/Guangdong 2 weeks ago & it was delivered to my designated NY area address mid-week (great) and signed for. Yet, to this date - there's no tracking info online and/or updated as signed for.
At about the same time, another small package with documents were registered mail with tracking # (RA sequence) from Hong Kong Post (postal) and delivered in about 10 days. Its movement/progress was tracked as it departed, sorting, arrival & zone to delivery - very useful & my "agent" can easily print proof on their end, without waiting for my SMS/email confirmation (at both USPS and HKP)
Funny thing is that both systems are integrated into USPS's online tracking but China EMS just didn't work, period - not the first time either.
As for choosing among the specialists, my understanding is - unless things had changed - for faster delivery, DHL is preferred due to their flight/delivery scheduling & networks abroad and rates a notch lower vs. FedEx or UPS (but with fuel surcharges likely going higher, if not already)
Ordinary airmail letters from here to Shanghai and other major cities generally will arrive between 7 to 12 days from the East Coast - the challenge is getting an official confirmation. Those "little" postcards used for proof for the US senders have a tendency to get "lost" or misrouted - using the online tracking system is generally adequate for me. It's just somehow, the USPS and China Post's synergy and integration of the tracking system aren't working well (yet).