The good thing about applying

is that they can use AP to travel abroad and EAD to work(which in your parents case is probably not necessary). So if your parents can get B1/B2 to come to US first, then file 130/485 in US, then do it that way, they can apply for AP at the same time. However, if they can't get B1/B2, then they have no choice but to file in China. While their case is pending, it is extremely unlikely that they will be granted any type of non-immigrant visa to come to the US before a decision is made.