Magnex and Janspeed are good performers but are more for the sutle driver who doesnt want the drone on the motorway or to be noticed that much while driving round town.
Scorpion and Piper are offer a slightly deeper note which also depends on the tailpipe option you go for.
For a really loud note then OMP and Ashley systems are the way to go more designed for fast road and competion use these offer a agresive note that some may find too harsh on motorway runs.
Best thing to do is get on you tube and punch in 'Saxo' and add some exhaust brands such as Ashley, OMP, Scorpoin, Piper, Madgnex, Janspeed etc etc and see how they sound.
Id all else fails - order the one that you like the look of the most then if its not loud enough and you want more noise then get a striaght through centre section from eBay or a online exhaust stockist.
Most people chooce exhausts simply on looks anyway :-)