I always get my Chinese Visa here in Auckland from China consulate general before travelling to HK. It takes only one week and your visa is ready. It's fairly a simple application form plus one passport photo and a copy of your ticket. For a single entry visa is usually cost NZ$60. I'm not sure how much would it be if you apply from China consulate in US. They usually offer urgent services too, you can get your visa in 3 working days but it apparently will cost you double!
I never tried to apply for a Chinese Visa to get into mainland in HK, but I think getting your visa from China consulate at your region is less hassle free.
Before travlling to HK, it's better to check it with the China consulate at the nearest place to your area in US. Maybe it costs you cheaper than applying in HK!
Hope this helps!