I own a Hellfire 60 and am going to get a Unibrow 60 to replace it but have not ridden one so bare that in mind when reading on. I would think they have a similar range in terms of the waves you would ride them in (chest to 1.5x overhead). They are both similar in terms of their rockers and both have similar and quite signifincant single to double concaves.
I think it would depend on the volume of the board compared to your weight and ability. The step down rail in the hellfire allows you to dig in the tail for good hold in better surf. The unibrow does not have a step down rail but then it does draw into a round tail which gives you good hold in better conditions.
Chris posted a really useful coment on the other thread in the Dan Mann section on the forum about the unibrow saying that if you are a bigger guy, and can therefore sink the rail easily on the unibrow, it can fulfil your needs as a step up/shortboard.
I would think that the hellfire will be more loose and pivotal owing to the diamond tail but the unibrow will have a more predictable fluid nature. I personally love the smoothness of a rounded tail rather than the snappy feel of the diamond tail so that is why I am selling the hellfire and getting a unibrow.
So in short I think they are silimar in terms of their wave range so it might come down to the kind of board/style you want to surf.