I've been wrong before. In my mind, it goes without saying that when I say something, anything really, it's prefaced by an imaginary "in my opinion."
It's hard to argue about maths, because you can be either absolutely right, or absolutely wrong. Sometimes I've made mistakes, and in those instances I'm absolutely wrong. More often than not, I get the maths right, and then I'm right.
But saying that something has X probability of Y occurring, is not the same as saying that X is best. There is no magical "mathematical formula" for creating lists. Math can inform decisionmaking, but it cannot make decisions. Never has, never will. You can try and incorporate as many different variables in as many different ways as you can, but you will never capture the subtleties of threat ranges, movement options, hell, base sizes, footprints, etc. It's almost impossible.
But sometimes you can find a truly "apples to apples" comparison.
The Fex Star versus the Tyrant Star.
So... this is a debate that went on and on between SinSynn and I back in the day. One of my favorite ever internet debates.
And at the end of the day, we left neither of us having convinced the other. But now it's a new edition, and a new debate.
So, let's clearly define our terms:
Contender A, the Tyrant Star:
Hive Tyrant, Devourers x 2, Tyrant Guard, 260 points.
Contender B, the Fex Star:
Carnifex, Devourers x 2, Tyranid Prime, LW/BS, 285 points.
As I understand it, Kirby has used the Fex Stars, and seems to think that the Fex Stars transfer well to 6th. I, too, have suggested that Fexes with Devourers make a great tool, once your HQ slot is used up.
But I would offer that if you are going to be using your HQ slots for Primes and attaching them to Fexes, that the Hive Tyrant with a Tyrant Guard is a superior option.
Combat Ability: Fex Star is the winner.
Durability: On its face, the Fex Star is the winner (one extra wound).
Heavy Armor Killing: When smashing, identical. If Smashing is not needed, the Fex wins. Usually. (See below).
Psyker support: Ohhhh wait. Guess the Tyrant wins this one.
Army support: I keep looking in the dex, I don't see any options for Preferred Enemy Bubbles on the Carnifex. Heh.
Vulnerability to Jaws: Tyrant wins hands down.
So for 25 points cheaper, I'm thinking I will be sticking with the Tyrants. The addition of the psyker abilities is huge. If I want to, I can splurge and grab a 2+ save, or get a Preferred Enemy bubble. *shrug* I like those options.