Yes I agree, such a tricky question.
You have to ask yourself, what events do I ride? If you don't race, then how long do you ride for? What terrain do you ride? How good are you at climbing, descending and bike handling?
I have both. They do different things pretty well. If I race in Sydney, then the dually works well (it is usually rocky). If I race in Canberra, then it is a 50/50 depending on the location. Other locations around Australia are the same.
With the hardtail, it
should be lighter than the dually (at least by a kilo, if you have the same components) and may be stiffer, so that can suit the shorter events. (up to 100km). Or where there is lots of steep climbing. Switchback climbing with limited gradient works fine on a dually.
You will find that there is no consensus though, and you will find that you probably will receive a 50/50 response for either.
As for my opinion, well, for the last 18 months I raced soley on a dually. It is great - 10kgs fast, resposnive etc. I recently got a hardtail - I ride it more, I love it for racing.
I also started racing in 1989 as a junior, when we had no suspension and toe clips were considered high-tech and before steel 'was real' - standard 'back in my day' square wheels story, so riding a sub 9kg carbon hardtail with suspension forks is F1-PRO in comparison.
The reason for going to a hardtail - I wanted the 20 second faster laps in a race. This works for me, and was important to me, your results may vary.
Also, what you get for your money will vary. $2500 gets a pretty good hardtail, but only dips into the lower end for a dually.
Talking to some mates who ride and race and own bike shops - this season, the hardtail is back in fashion, everyone who bought a dually learnt how much a bike could be pushed and ridden hard. These skills transfer nicely over to the HT. Plus you are stronger for having pushed the heavier dually around.
There is no correct answer......