Fonsini,
I think Beagle above has given a pretty comprehensive and accurate answer . I have 100+ hours on the Gnat mainly as a student in the 60's and 800+ hours on the Hawk mainly as a TWU instructor.
The Gnat was significantly faster than the Hawk, had ultra-responsive controls including a head banging rate of roll and was more compilcated . It was thus a challenging aircraft to fly and therefore, in my opinion, a better lead-in to later fastjets. The Hawk was less of a challenge and, because of this, the TWU course had to be altered to give students a similar degree of challenge to the Hunter course that it replaced.. Its performance at low-level in the TWU environment was always a problem - a realistic top speed of about 480Kts thanks to the external gun pod and wing hardpoints and sortie lengths of less than an hour. Beagle has detailed the systems and equipment but I would add 2 others. The back seat for we instructors was luxury compared with the contortions necessary in the Gnat. The other is the simplicity of arming the Gnat's ejection seats. No pins to drop just a ball on a stick in the back of your neck that you turned through 90 degrees to arm it.
Which was the better? The Hawk was simpler, more reliable and a quantum leap for the instructot. The most fun? The Gnat by a country mile.