Arbor Scientific Item # 94-1970 is less expensive than either the Go Direct® Projectile Launcher (GDX-PL) or Vernier Projectile Launcher (VPL). Advantages of the Vernier options include:

-More reproducible results because the Vernier version is pneumatic. A spring will deform over time, and can behave differently in each trial.
-Accessories such as the Independence of Motion Accessory (IOM-VPL) allow you to do more experiments.
-Two photogates built into the barrel allow excellent measurement of the launch velocity.