The first time I attempted a 24, I was on my Anthem dually, the second one was on my XTC hardtail, running as a single speed. All things considered, I actually enjoyed riding the HT SS for 24 hours a lot more than the dually.
On the dually I was overthinking things, when to lockout, if I was in the right gear, should I grind or spin up hills, whats that funny noise from the suspension? my gears are playing up, etc etc.
On the HT SS, its simple. Ride, eat, ride, drink, ride some more, eat, ride a bit more, drink pedal pedal pedal......
My choice would be go the HT, ditch the gears and enjoy the experience...
Oh, and the sore back comments...I find on the HT I can't ride lazy like I do on the dually. On the HT I am forced to stand up through the smaller bumps that I wouldnt bother standing for on the dually, and it is these smaller bumps that you think are being absorbed by the suspension that cause me grief later in the day. The dually feels better at the start of the race, but I end up suffering for being lazy and trying to conserve energy 8 or 9 hours later.