“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.” -Romans 3:29–30
The person of Christ was the plan of God for the whole world from the beginning of time. Peter states this clearly in his first epistle.
“but with the precious blood of Christ, like that of a lamb without blemish or spot. He was foreknown before the foundation of the world but was made manifest in the last times for the sake of you who through him are believers in God, who raised him from the dead and gave him glory, so that your faith and hope are in God.” -1 Peter 1:19–21
Therefore, God is not the God of the Jews only, but the God of the Gentiles also. Paul has already treated the privilege of being a Jew and clarified that theirs was a special inheritance as priests to the world but that righteousness is obtained by all the same way, by faith alone in Christ alone. Paul applies this very truth in response to the racial conflicts in the church at Ephesus.
“For he himself is our peace, who has made us both one and has broken down in his flesh the dividing wall of hostility by abolishing the law of commandments expressed in ordinances, that he might create in himself one new man in place of the two, so making peace, and might reconcile us both to God in one body through the cross, thereby killing the hostility.” -Ephesians 2:14–16
Paul’s last statement in verse thirty has been the subject of some debate, namely what is meant that God, who is one, will justify the circumcised (the Jew) by faith and the uncircumcised (the Gentile) through faith. He is saying essentially the same thing. There is nothing in the syntax that suggests a difference in how justification is obtained. By and through mean the same thing in this context.
There are two potential explanations, however. One is that Paul is conscientiously varying his word choice for literary purposes (so his writing doesn’t sound awkward). Though it’s probable, I’m not convinced of this argument. The other is that Paul is accommodating the Jewish position to explain how they obtain justification by faith, meaning more directly, because of their privileged proximity to God as a priestly nation; and, the Gentiles obtained justification through faith, that is indirectly, through the priestly work of the Jewish people (they are the physical, human source of Jesus the Christ). And by stating this way, he is only reaffirming more precisely what he has previously argued via syllogism.
Regardless, at the end of the day, the expressions mean essentially the exact same thing: justification is by/through faith in Christ for all who believe.