Generally speaking, American immigration lawyers are more professional, experienced and sympathetic. At least my experience is so. Although they are relatively more expensive than Chinese lawyers, but you can expect better work from them. Another reason, they can chat easily with the immigration officers!! so that your case will have better chances to be approved in a more relaxing atmosphere.
If your case is quite complicated, I also suggest you to hire an American immigration lawyer. They will save you from possible delay or even denial.
No intention to offend Chinese immigration lawyers. There are also Chinese lawyers who are very responsible and professional. But remember to ask your friends who did case with them first before making any decision. Interview the lawyer is also critical.
Good luck to you all.