They are both pretty popular. It mostly depends on when the church launches. Many churches that launch in January, choose the 12-month plan and go for a whole calendar year. While many that launch in August or September opt for the 9-month plan and follow the school year. While less popular, some churches even launch at other times during the year or choose to run a season that is ten or eleven months long.
There is not a right or wrong choice or best practice here, it’s more about what fits your church culture and calendar and which option accomplishes what you wish to accomplish.