I think mechanical pencils are better to use because you do not have to get up to go back and forth to sharpen them.  The average pencil can be sharpened 17 times.  Not having to sharpen your pencil saves time and allows you to get work done faster.  Since you can refill the lead, you can use the same pencil all year long as long as you have an extra eraser.  You need trees to make pencils, but mechanical pencils can be made from recycled plastics.