Well... 650S is in different sector compared to 991TTS. one is super car, and the other is almost like GT sports car. 

The appearance pretty much separate two cars.  The powertrain is also very different; 560hp/650hp

I see TTS has only 1 advantage; back two small seats.....  but believe me if one has money to purchase 650S, they are going to have two other cars to drive when back two seats are needed.... 

Daily driving ability.... 650S drives very smooth in the city, so does TTS. 

650S is more eye catching, and TTS with 200K price tag nobody really looks at this car..... 

Over here in Taiwan 650S is US$510,000.00 no option, TTS: US$329,000.00.  Two very different sector of cars here. 

I think if TTS can throw in 620hp (as GT2RS) with a very slick look (very different look than 911s), then Porsche will be successful, as it will be start eatting up R8/ 458/ Hurcan/650S market for a cheaper price....