I've heard Comptech, Anytime, Flowmaster, Magnaflow, and RM exhausts on a NSX, but I personally think the HKS exhaust is the "more quiet" one out of the bunch as it still sounds throaty but is more mellow. I haven't heard the RSR exhaust on the NSX but I've heard RSR exhausts on other cars (Preludes, Civics w/B-series, Integras) and it's also very quiet and mellow sounding, similar to HKS. I used to have a Magnaflow muffler on my NSX and oh man, it was so loud when driving that I really didn't care for it that much. Although I would have to admit that it sounded nice in 3rd gear flying by people on the freeway with the windows rolled down :biggrin: .
For my HKS exhaust, performance-wise, I doubt I gained much extra power out of it but the main things that I wanted out of a good exhaust on my NSX was a good name (good quailty welds & piping), something mellow but louder than stock, and something that looked clean when on the car (and the name 'HKS' sounds cool too :biggrin: ). If I didn't find a HKS system, then I would've gotten the RSR one from a guy here locally when he was selling his.