I went for the Sport Powervalve (R32 style). It's tasteful and refined when you're doing town driving, and sounds pretty much how you described it. You get all the burbles and grumbles on the over run but it's not at all ott. It's deep, but with a 'soft edge' to the note, if that makes any sense.
It's noticably deeper than the stock system, but in a way that you don't feel like you're attracting attention rolling through town.
On boost going through the gears it's a different thing completely.
I did consider the quieter Prestige option, but I'm glad I went with Sport. I'd imagine Nige could swap over the silencers either way if you feel you want it louder or quieter after a while.