The Scorpion system really does seem to be the most popular about going by recent members and queries. Any videos or sound clips ive heard of the Scorpion system all sound great though. Even with both cats kept in, any aftermarket exhaust is going to sound louder than standard... It just depends how loud you want it to be!
I for one had the KTec system along with decat, recently switched to Akrapovic as the KTec was way to boomy and loud for me.