Not an easy question.
It depends on what your needs. If you're looking for ultimate performance and power, the 996TT is the way to go, withouth spending so much money, you can turn a 996TT into a wild beast with more than 500 reliable HP.
If you're looking for latest tech gadjets, "new retro" design, 4 years of free headackes/warranty and a top model looking car, 997 is the way to go.
Personaly I find the 997C4S cabrio the best looking and sharming 997, it's hard to resist at such beauty, but "power" is my midle name
... only after a test drive in both I would have my mind clear.
Have already drove several 996TT and non 997C4S.
J.Seven