The right choice for your lab depends on the activities in your curriculum. All of the activities in Physics with Vernier can be performed with the 1.2 m track, including two-cart collisions.
The 2.2 m track allows for a cart experiment to last longer and makes for impressive demonstrations at the front of the room. It can be more difficult to store and to move between classrooms.
All Vernier carts, including the Motion Encoder Cart and the Motion Encoder Receiver can be used with either the 1.2 m or 2.2 m tracks.
If your motion detectors are the very old blue models that do not measure objects reliably closer than 45 cm from the detector, the long track will give you more space for experiments. Modern motion detectors measure objects as close as 15 cm, so while you still need to be careful, there is enough room for good collision experiments. It is also helpful to use the Motion Detector Bracket (DTS-MDB) to position the motion detector farther from the end of the track. If you are not sure of the capabilities of your motion detector, see Which Motion Detector model do I have?