A Milltek cat-back like that one is about £400 new isn't it? It's certainly not a huge amount more. Would definitely recommend the non res one. It's not chavvy and sounds a whoooooole lot better than standard when you're giving it the beans!
The Pipewerx DP and sports/de cat is supposed to be fairly faultless, but unfortunately one of the more expensive options. It's what I want when I have the money (to go with my Milltek cat-back). Either that or wait for the XS Power one which should be ready any day now, almost identical but cheaper.