I can imagine the dilemma... Its why I'm hoping to run a tyrant, a tervigon and 2 mawlocs or trygons... (mawlocs probably due to cheaper cost) (2k)
apart from the tyrant, im gonna be honest and say that although they are scary combat creatures, none of the others pack the same sort of punch as more dedicated combat specialists. The only thing with mawlocs is that when using their burrow ability they arent on the board - and as a result the enemy wont have as many targets, so this might make your other big creatures suffer. Of course, this could be viewed in the other light that - the enemy will attack the other creatures when mawlocs arent about - but the mawlocs when they are on the board - wounding all of them, but killing none.
I think that of all the beasts, people will, in the short term at least, be paying much more attention to trygons / mawlocs simply because they are BIG models! the payback for killing a tervigon could also be worth paying attention to them. However I think we wont see too many of these about until an official model is released (FW i'd presume closer)
However, I am also debating the carnifex. it seems to have fallen out of favour now that it is more expensive and there are other toys to play with. Its ability to be taken in a squadron could be really exploited... two or three of these rushing across the board simply could not be ignored, they are, after all, still the strongest unit and thus also one of the best anti tank weapons. However, if they are added to the dilemma of tervigons and tyrants then the enemy is never going to be able to stop everything!