In the case of those two denominations, the differences are much larger than the very minor theological differences between the Westminster Standards and the Three Forms of Unity. The RCA is a mainline denomination that is at least moderate to liberal in its orientation, although there are still conservatives/confessionalists in there, such as Kevin DeYoung. The PCA, on the other hand, is a conservative reformed church body, although it is kind of a big tent with a lot of diversity. In the PCA you will find confessional reformed and broad evangelicals, but overall not much in terms of liberal Christianity.