Hall effect vs Current Transformers as Current Sensors
Yes, Hall effect sensors sense the magnetic field caused by current, and can therefore measure absolute current.  A current transformer can only sense current down to some minimum frequency below which gain falls off rapidly.  A current transformer can't sense a fixed current.

If you are measuring current of something that is inherently AC, like the power line, then a current transformer can be appropriate.  If you really need to sense DC current, then you can't use a current transformer and a Hall effect sensor may be appropriate.  Keep in mind that a Hall sensor requires separate power to operate.

The other major way to sense current is by using a sense resistor.  The resulting voltage is not isolated, so that may need to be dealt with.  In the case of a sense resistor, it is often convenient to put some detection and processing circuitry together with the sensor.  That can the convert the result to digital and send the digital information over some isolation barrier, like a opto-coupler.