Offensive systems depend on the placement of the setter. A coach chooses a system depending on their setter's attacking abilities (or lack thereof) and the strength of their attackers.
The 5-1: One setter that goes all the way around the rotation, so you have the same setter the entire game. A 5-1 setter normally needs to be able to block and attack. When running a 5-1, there are only two full-time attackers for three of the six rotations (which can be disadvantageous).
The 6-2: Two setters, and the setter is always in the back row. This rotation can be advantageous because there are always three attackers in the front row.
The 5-2: One setter always sets in the front row, one setter always sets in the back row. This creates more variety in plays and strategies.
The 4-2: This system is best used for young athletes just starting with the game. The setter is always in the front row to eliminate excessive running and confusion.