Well, for you're situation, I think it would be best if you gave us maybe a copy of the list yo ussually run or what you already have, as both serve relatively different purposes, (the tyrant being mainly anti infantry and light tank while the carni is obviously ong range tank destruction as well as anti infantry)
If I had to have my pick, I think that I would pick the tyrant as it is more destructive and all around more of a flexible thread, assuming that you already have a sniper fex or two in your army to handle the heavier tanks. If not, and the list already has anti personel in abundance, I would definately go with teh sniper fex.
Both are rather flexible units that can serve more than one purpose, but it depends what will work the best in you're army IMO.
Warrior