Honestly, I don’t think the answer can be distilled down to a simple ‘better’.
Each one has their strengths. Just like each of the 12++ variations of even the 911 (Carrera, Targa, Cabrio, S, 4, 4S, GTS, Turbo, Turbo S, GT2, GT3, RS) has its focus areas making it better suited for their specific target audiences.
The 911 is a better car in many way. Clearly better utility as a daily driver as it has +2 back seats. I use those everyday to drop off my kids at school. It’s a better more substantial GT style car as it has more standard features and a deeper option list.
I’ll give you that from a handling stand point, the Cayman/GT4 series has better idealistic handling/balance qualities. The 911 is a not a traditional handler in the classical sense with its extreme weight bias. Drivers even have to apply a distinctly different driving style for the 911.
The 911 rear engine style is not all negative and can be exploited, especially under race conditions. Because of the weight bias, it has better traction under acceleration with more weight over the rear powered axle, and better traction under braking with weight to all 4 wheels. In racing, passing is done more often under braking and acceleration than anywhere else, as the ideal handling line in curves is limited.
And of course the better packaging advantages of enabling rear seats in a rear engine car.
