The 9v9 is a miniature version of the real 11v11 game. Of course, the overall game is missing 4 players, but there are no build-out lines.
At 7v7, we recommended a 1-3-2-1 because it supported the build-out line mandate, as well as helping to prevent players from booting the ball because they have more help around the ball.
Into 9v9, there are a lot of choices.
1-4-3-1. This would be a natural progression from 1-3-2-1 because you would be adding a central defender (#5) and a central midfielder (#10). The benefit to this progression might be that you have players who rotate between those positions. Players familiar with the demands of those positions will help with the step-up to the bigger field with more players.
This choice of formation leaves the 1-3-2 in place from the 7v7 formation, but because the nuances have changed, you might want to add numbers further forward. In this instance, it would be to add a #7 and #11. Now you can focus on using more width in attack, as well as positional relationships in diamonds and triangles.
Other formations to consider and explore: