i'm surprised nobody's said it yet....the 04's had 410 gears, and the 05-06 have 456's ...noticeable enough for me, anyways...personally, i think the qc's ride a bit stiffer then the rc's...i prefer the look of the 06's interior better, maybe it's just the nav/stereo head unit that's much better...honestly, they both are designed to look good and haul ass...so you'd just have to drive them both back to back imo, to get a good feel