Looks? Its a coilover, you cant see them ........
My personal experience is that you WILL notice the difference between D2's and HSD's, the former have harsher damping and can be quite choppy down a bumpy road, where the HSD's are much more compliant. Cant speak for BC's, KW's or Ohlins but I also know the differences become a lot smaller as you spend more money
What's your take on true coilover vs OEM setup coilover in the rear?