We have adopted and are adopting from RUssia, so I can't help too much with agencies that work in China, but have you read the suggestions that are pinned at the top of the bb? I think you'll find some good general helpful hints there.
For me, I made out a spread sheet to compare agencies when I was choosing. across the top of the page I put the agency names and down the side, details about each one. Like cost, projected time frames, age ranges they believe they can refer you to, travel expectations, humanitarian work in the country, how quick they were at responding to initial requests for info and follow up questions, and comments from references, etc. This has been a helpful strategy for me with lots of this "how do I pick" situations!

I just did this a few weeks ago with choosing our dd's preschool for next fall, and looks like we'll be doing it all over again if my dh gets relocated
Best wishes with this part of your journey! Ask as many questions as you need of each agency. You'll find that as you gather your information, you'll somehow get a gut feeling of who you should work with.
All the Best,
Lynda