|The Names printed on the sensors
The Line Follower has: LineLeader-v2
Where as the Light Sensor Array has: LightSensorArray
|The physical form factor
The physical form factor is same for both sensors, however the LineLeader-v2 has bright Red surface, where as the LightSensorArray is Black.
Internally their processors and firmwares are different, the NXT-G blocks and NXC/RobotC API's are also different. (Note, they don't work with each other).
The LineLeader-v2 sensor internally performed the line following computations, and provided simpler output to follow a line.
The LightSensorArray only provides 8 numeric readings for 8 sensors. It does not perform any computations internally. It is somewhat faster than the LineLeader-v2 in reporting readings. You have to write your own line following algorithm.
The LineLeader sensor was not acceptable in some RCJ competitions because it was too easy to use due to it's internal PID algorithm.
The LightSensorArray is acceptable in RCJ competitions. (Note, it does not have PID algorithm, if you need to follow line, you will have to write your own line following algorithm).