The old which is better debate
To be honest, you wont find an answer to that one... Both superchargers and turbochargers have worked very well on the Saxo... but at the same time, both of these have also blown Saxos up!
The supercharger is probably still a little more developed than the turbo kit... but turbos are more fun in my opinion.
Swings and roundabouts really... bottom line being... They both cost a lot of money and will need more looking after once fitted! If you really want more details I will try to answer all of your queries...
T4 is bigger than T3, which is bigger than the T2. You can also get turbo units inbetween the sizes... eg T25 (read T2.5). Added to this, you can also get Hybrids... eg, A T2 turbo housing, with T25 internals...
The list of options is almost endless!