I tend to think that the 6.3's Hot Rod reputation was obtained a very long time ago, when the car was new, and the buying public had been shocked by the huge power difference between the 6.3 and the smaller engined models. Also, well before the 6.9 broke cover.
Move the time factor onto today - some almost forty years later. The Hot Rod notion attributed to the 6.3 originally is now only just that - a notion. Of all 6.3's I have driven during the last two years, none met the expectations one could have expected. There is no doubt that the 6.3 drives differently to a 6.9 - it is a bit like a Diesel locomotive - a driving characteristic that I would attribute to the four speed gearbox. Well, it is a four speeder, but you would not want to engage the low 1st gear for standing starts - in my opinion, nowadays the drive train is too fragile to cope with that low gearing. If things break in the rear end (which they are inclined to do), try buying the spares required, and the time factor involved in replacing broken parts. Incidentally, 1st gearing is low enough to enable you to virtually come to a full stop when approaching traffic lights, and you only have to use the brake to stop the car from rolling forward. It is now old technology.
So, let's turn to the 6.9. In my opinion, nowadays 6.9s are every bit as quick as the 'Hot Rod' - even from a standing start. Respect for machinery and fragility have always prevented me from perhaps using the low gearing of the 6.3, and certainly once you get moving with the 6.9 from about 30 km/hour, it's really all over red Rover - pardon the pun. That comparison would certainly be applicable if I use my "Gleaming Beauty" as the yard stick. We should also touch upon ease of ownership as far as maintenance and every day useage is concerned. On that score, the 6.9 wins hands down. Sure enough, the hydraulic suspension factor can constitute a draw back factor but then, on the other hand, the Air Suspension featured on the 6.3 can also leave you breathless. Labour wise, try and replace the front valves, as well as the main control valve.In comparison, similar replacement procedures on the 6.9 are almost child play.
The fuel system on the 6.9 is much more user friendly, with its well proven K- Jet system that's identical to the 450 system. Plenty of second hand items available, unlike the old mechanical injection system and hugely expensive injectors as found on the 6.3. Then try and remove/replace those components on the older model - it will raise your blood pressure to unacceptable levels. In closing,and referring to every day user friendly operation, which is the better car ? In my opinion, the 6.9 wins hands down from just about every perspective. Regards. Styria
N.B. Anthony, I am happy to hear your perspective on this subject matter.