I still have a Blackburn Mars 3 at home, as well as a few Superflashes. Short answer: the Mars 3 is stored, the Superflashes are used.
– The Superflash is slightly more compact. It's especially thinner, so if you attach it behind a rear rack, it takes a bit less space.
– The collar around the Mars 3 might give one more flexibility. However, the bracket is less sturdy. I had two Mars 3 behind my trailer, but lost one because of vibrations, I assume. Other down aspects: the collar is hard to fix, the light can't be bolted in place.
– In steady mode, the overall brightness is fairly similar. When I used it daily, the rechargeable batteries used to last less than a week, whereas they last 1-2 weeks in the Superflash. I assume that the 7 LEDs end up using more than 0,5 W.
– In spite of the tiny yellow LED of the Mars, the side illumination is fairly similar Ditto for angle illumination.
– In flashing mode, the Superflash is definitely better. Generally speaking, I would NOT use the Superflash in flashing mode at night, but I do use it quite often in fog, sun-in-my-face or other low-visibility situations. In these situations, the Superflash definitely wins.