For a "road" car, the 6"diameter (152mm) system is the best-sounding exhaust I make for the Toyota-engined models! When the VVTi kicks in, the scream is sensational. You can drive through town/suburbs with no problems - it's not an obnoxious sounding exhaust - and it doesn't drone at all.
If you are a trackday enthusiast, or do a lot of motorway miles, you'll need the quieter 7"dia system - still a great-sounding exhaust but doesn't scream so much.
If you run at Bedford (UK track, very strict on NOISE) you will need the bigger, even quieter 8"dia. This doesn't scream but it does have an addictive howl on the second cam!
Twin tip layout - option of plain/slashed or rolled edge. Some owners have opted for the 6x4 oval from the supercharged cars - it can be fitted, provided you trim the diffuser exit to make room for it.