There are two major views for the destination of Paul's letter to the Galatians: the Northern and Southern views. I understand that these views impact the possible timing of the letter, but what impact do they have on the interpretation of Paul's letter? Do commentaries following one view or the other differ greatly?
At issue is the ambiguity of the phrase:
To the churches of Galatia:
—Galatians 1:2b (ESV)
The text of the letter shows that Paul had been with them recently:
I am astonished that you are so quickly deserting him who called you in the grace of Christ and are turning to a different gospel—not that there is another one, but there are some who trouble you and want to distort the gospel of Christ. But even if we or an angel from heaven should preach to you a gospel contrary to the one we preached to you, let him be accursed.—Galatians 1:6-8 (ESV)
Naturally, interpreters have tried to identify which region Paul had visited. Luke records:
And they went through the region of Phrygia and Galatia, having been forbidden by the Holy Spirit to speak the word in Asia.—Acts 16:6 (ESV)
At the time, Phrygia was an ethnographic region and not a province or other political designation. Paul and his companions had recently left the prominent cities (Lystra, Derbe, and Iconium) of the southern portion of the province of Galatia, so in this context Luke is talking about northern Galatia. This visit to Galatia occurred during Paul's second missionary trip.
Northern Galatian Theory
Luke makes clear that Paul's trip to the ethnographic (Northern) Galatia occurred after the Jerusalem Council. That practically forces us to identify the meeting of Galatians 2 with the meeting in Acts 15. More than that, the recipients of the letter would have heard the decision handed down by the apostles in Jerusalem freeing Gentiles from observing Jewish customs and likely have seen a copy of the letter explaining the decision. Then some other group (often called Judaizers) would have taught "a different gospel". When Paul heard about it (probably during his third mission), he responded with this letter.
Southern Galatian Theory
On the other hand, if Paul was referring to Roman province, he could have been writing to the churches he and Barnabas established in Iconium, Lystra, and Darbe. This first trip was described in Acts 14 and therefore predates the Jerusalem Council. The visit to Jerusalem in Galatians 2:1-10 would correspond to Acts 11:27-30, so the issue of whether gentile converts would need to be circumcised was still undecided when Paul first visited the Galatians. This would allow Paul's letter to have been written in response to the events described in Acts 15:1-5 that precipitated the Jerusalem Council. Paul may even have written the letter from Syrian Antioch before leaving to attend the Council. Or the letter might have been sent by Paul to accompany the letter circulated as a result of the Council.
In some ways, which theory you pick results in only minor differences in interpretation of the letter: Paul clearly opposes forced circumcision. But when it comes to reconstructing a biography of Paul, the two theories suggests two radically different people. Galatians and Acts are our best sources for information about the life of perhaps the most important Christian author, missionary, and thinker of all time, so it's critical to reconcile the accounts if we can.
The Northern Galatia Theory paints Paul as somewhat of a maverick. He knows that the first generation Christians support his position and yet he does not lean on their decision. He's even a little dismissive of their influence:
And from those who seemed to be influential (what they were makes no difference to me; God shows no partiality)—those, I say, who seemed influential added nothing to me.—Galatians 2:6 (ESV)
In contrast, the Paul of the Southern Galatia Theory is less self-assured, while still being convinced that God does not require gentiles to follow Jewish customs. In this case, the resolution of the controversy is still fresh or, perhaps, has not even occurred. We get the feeling that Paul is still working out the issue as he writes the letter:
I went up because of a revelation and set before them (though privately before those who seemed influential) the gospel that I proclaim among the Gentiles, in order to make sure I was not running or had not run in vain.—Galatians 2:2 (ESV)
I tend to agree with Daniel B. Wallace's argument for the Southern Galatia Theory. As a result, I think Paul is somewhat more sensitive to cultural issues (such as head-coverings) than he appears. This version of Paul is a touch more humble and less dogmatic than is popularly understood. There is not as much difference between Pauline Christianity and the faith of Jesus' followers during His life as some people assert.