Best exhaust noise: GTS
Best engine noise: CSL
IMO, you can make a standard e46 m3 perform CLOSE to as well as the 3.2L CSL; upgrade to the CSL intake/headers/seats to reduce weight/body kit, and if its a ZCP, you get the same steering ratio/suspension/brakes.
The GTS, however, is hard to replicate due to its 4.4 liter unless you upgrade to the DINAN 4.6L engine ($26k)
With that being said, I would take the GTS over a CSL.