What is the difference and membership between the two groups?
This article is helpful on that-
http://theaquilareport.com/13-differences-between-the-pca-and-the-pcusa/
PCUSA has 1,760,200 members(according to wikipedia)
PCA has a total membership (communicants, ministers and non-communicants)– 367,033, an increase of 3,014 per the last GA report
PCUSA is much larger (about 1.7 million), but has been losing membership for 50 years. Currently losing about 5% a year. They ordain women and practicing homosexuals. They are a post-Christian mainstream denomination. They have a ‘Book of Confessions’ from which leaders can pick and choose. They have abandoned the Presbyterian form of government in favor of a hierarchical governance.
The PCA is smaller, with about 300,000 members and has been fairly stable in numbers over the last few years after a few decades of rapid growth. It holds to a traditional view of marriage and will ordain only males. Leaders are required to affirm the Scriptures and subscribe to the American version of the Westminster Standards. It is generally considered to be ‘moderate’.
Although the PCUSA is much larger in numbers, I’ve seen speculation that the average Sunday attendance of the two is about the same.
The PCA fields many more foreign missionaries than does the PCUSA.
And finally, if a congregation wants to leave the PCSUA, the denomination will generally either take their property or extract a heavy cash settlement. PCA congregations have to follow a formal series of rules, but can freely exit with their property should the congregation desire to do so.