I think Paul helped spread Christianity more because he spread the gospel to the Gentiles.
How did Paul spread Christianity?
Paul, as an apostle, impacted the spread of the good news of salvation through his teachings and by writing a lot of letters. Paul aided in expanding the church by traveling to different cities, towns, and regions by road and sea, flinging open the doors to Gentiles.
Paul strenuously fought for his conviction that the gospel was for all people and that no barriers should be put in the way of Gentiles.
For Constantine, his decision to cease the persecution of Christians in the Roman Empire was the core reason for his recognition as helping to spread the gospel. He stopped criminalizing Christian worship in the entire empire of Rome.