What's in a Preposition?

I need everyone to sanity check my exegesis here. Am I reading to much into this? First, I look at Romans 3:27-30:

Then what becomes of our boasting? It is excluded. By what kind of law? By a law of works? No, but by the law of faith. For we hold that one is justified by faith apart from works of the law. Or is God the God of Jews only? Is he not the God of Gentiles also? Yes, of Gentiles also, since God is one—who will justify the circumcised by faith and the uncircumcised through faith.

Question: What is the difference? Why the selection of two very different prepositions here?

The circumcised are saved by their faith. The uncircumcised through their faith. To me, the implication of this passage from Romans about circumcision and faith has a typological similarity to Mark 16:15-16:

And he said to them, "Go into all the world and proclaim the gospel to the whole creation. Whoever believes and is baptized will be saved, but whoever does not believe will be condemned.

This looks like two sides of the same beautiful coin. On the one hand you have those who were divinely adopted that are saved by faith. On the other hand you have those who were divinely converted that are saved through faith.

Is this an exercise in hair splitting? I'd love to hear some thoughts on this.