i think the stealth is a little more refined and is ridden a little bit longer than the Hellfire. I do think, however, that they can and should be ridden in similar conditions. I think the major difference is the stealth will feel overall like a flattened shortboard while the Hellfire has, in my opinion, a more attractive curve and distribution of foam. You get great response and versatility.
That's my take, but I don't surf t he Stealth much, it just isn't one of my favorites.