Ive got decat and non resonated Milltek, it has no drone and is actually still pretty quiet if you want it to be, only when your on it it sounds mad! I get a cool popping off it when the traction control kicks in, id imagine thats what you'll get from DSG too. Its cool.
Couldnt comment on power/ noise reduction between decat and sports cat though.
For the sake of changing it once a year im happy with decat. If a sports cat loses its peformance in way of failing emissions, which i have seen, then its a very expensive replacement.
Just go decat and swap it for mot once a year!