Question:
What is the difference between someone singing a solo/duet/trio and having a choir sing a selection during a gospel service?

Answer:
Instead of trying to answer that question directly, let us see if the Scriptures have anything to say about smaller groups of singers performing for the entire congregation. In Exodus 15, “Moses and the children of Israel” sang the song of triumph: later in the chapter, Miriam and the women of Israel sang a responsive type of song, with the women singing the verses and Miriam singing the chorus (verse 21; similarly 1 Samuel 18:7). In Judges 5, Deborah and Barak sang a duet. In 1 Chronicles 25, families of the Levites were appointed as temple musicians. In the New Testament, the disciples gathered at the institution of the Lord’s Supper, concluded by singing a hymn together (Matthew 26:30). In the early assemblies, the singing of psalms and hymns was clearly used as a means of instruction as well as worship: “Teaching and admonishing one another in psalms and hymns and spiritual songs, singing with grace in your hearts to the Lord” (Colossians 3:16). That’s why we learn to sing so many songs and choruses in Sunday school. It is easier to remember Bible verses and Bible truths if they are linked with a particular melody and rhythm. In Revelation 5:8, 9, the 24 elders (considered to be resurrected saints) “sang a new song.” Also in Revelation 14:3 and 15:3 we find large companies singing together.

Putting all of this together, there was much congregational singing in the Old Testament, though not exclusively that; and the New Testament pattern seems to be almost exclusively congregational singing, or else a singer teaching hymns to the rest of the congregation. There doesn’t seem to be any definite scripture either forbidding or encouraging the use of small singing groups in certain meetings such as gospel services, so we want to avoid being legalistic one way or the other. The important thing is that the consciences of all in the assembly be considered and that the matter not become a source of strife and conflict in the assembly. It would certainly seem to be appropriate for a soloist or small group to teach the congregation new songs that have never been sung before; but again, there should be agreement in the assembly as to the appropriate time and way of doing this. “Let all things be done decently and in order” (1 Corinthians 14:40).
Something to consider with regard to having small groups sing at gospel meetings: while the music may be beautiful, it is often difficult to understand the words when others are singing. So at the very least, copies of the words should be handed out to the congregation. On the other hand, congregational singing forces, or at least encourages, each one to look at the words in the book, so the Holy Spirit can work through the “eye-gate” as well as the “ear-gate.”