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