Probably not much difference btw the two, all things considered. The Unibrow"s volume tapers from under your chest quite quickly fore and aft, making the board more instantly reactive and sensitive to subtle movements. The Hellfire carries it's volume further back towards the tail making it a little easier to catch waves. To me the Hellfire, in the 6/4 and with less actual volume than the 6/4 Uni, still "seems" to have more volume and be more stable because the volume is carried out to the rails with a flatter deck and less tapered rails; the Unibrow's deck being more domed. I also think the Unibrow has a little more tail lift which should equate to easier, more natural vertical surfing.