Motion Detectors and photogates can both measure motion, but both have advantages for certain experiments.

PHOTOGATES
Photogates function more like a spark timer in that the only information collected is the time at which a specific event occurred. Depending on the photogate timing mode being used and the object passing through the gate, other measurements such as position, velocity, and acceleration may also be calculated. Because of the sparse amount of data, students need to have a deeper understanding of the math behind an event in order to analyze the data. The age and ability of the student determines whether this is a pedagogical advantage or a disadvantage.

Experiments that use Photogates:
http://www.vernier.com/publishers/probecorrelation.html?sensor=VPG-BTD

MOTION DETECTORS
Motion Detectors present a more comprehensive record of the experiment since they can produce real-time graphs of an event. For example, when a student walks towards and away from a motion detector, they see instantly that moving away from the sensor produces an increase in a position vs. time graph. This added feedback is especially powerful for visually-oriented learners, and those with fewer math skills. However, motion detectors can produce noisier data due to sound reflecting off of something other than the intended target.

Experiments that use Motion Detectors:
http://www.vernier.com/publishers/probecorrelation.html?sensor=MD-BTD

ADDITIONAL INFORMATION
The area of detection for a photogate is much smaller than that of a motion detector, which can help to focus the data on only the event of interest. As a result, there are some events that can only be measured with photogates such as smart pulley (rotational data) and projectile motion with an object such as a marble.

Our starter package for physics includes only the motion detector, while our deluxe package includes both a motion detector and two photogates.

OTHER OPTIONS
Two other options for measuring motion are the Motion Encoder System and the Go Direct Sensor Cart. The data reported by these dynamics carts are similar to the data reported by a motion detector.