Okay, here is some optimization for you. Rockgut trolls aren't that much worse in close combat than fellwater troggoths. They actually get better and better compared to fellwater troggoths, depending on the opponents armor save. At 3+ save, the two types are equally good, and at 2+ the Rockguts are actually better.
Their defensive abilities comparative strength are negotiable, and I guess it depends on what's hitting the trolls. Same goes for the ranged attacks.
What I'm saying is this - People have been bashing the Rockguts a bit, but I see them filling the role of armorbreakers perfectly well in a troll army. My spidersenses is telling me to balance a troll army around 2/3 Fellwater to 1/3 Rockguts against 'all comers'. When you meet Nurgle, witches and death, you would much rather have fellwater troggoths, which is why I'm leaning in that direction.