Difference between gyro sensor and compass sensor
I haven't personally used the HiTechnic sensors, but the difference between a gyroscope and a compass is that one measures absolute orientation and the other measures change in orientation.

The compass measures absolute orientation via a magnetometer that measures the effect of the Earth's magnetic field on the sensor. The gyro sensor uses some form of MEMS to detect changes in orientation. The compass also only works in two dimensions, while the gyro likely operates in all three dimensions of orientation.

You might also be interested in this answer from electronics.se which includes more detail than mine and also compares accelerometers to compasses and gyros.