The main different between the two is that the Panamera has a dual clutch transmission instead of an automatic like the RS7. So which is faster?
The RS7 hits 60 miles per hour in 3.2 seconds and flies through the 1/4 mile at an impressive 11.6 @ 120.4. MotorTrend tested the pre-facelift Panamera Turbo which hit 0-60 in 3.5 second and the 1/4 mile 11.9 @ 114.7. The facelift manages 0-60 in 3.4 seconds and firstname.lastname@example.org. The extra 20 horsepower definitely shows adding approximately two miles of an hour trap speed. The RS7 is the quicker and faster car however and it will take the Porsche Panamera Turbo S with its 570 horsepower rating to challenge it.
Now which car is more playful? Audi's notoriously suffer from understeer due to their nose heavy designs and the RS7 is not much different. It also understeers noticeably. The Panamera suffers from much less understeer and has a more precise steering feel. It's simply more fun to drive with greater feedback.
Both of these sedans are spectacular cars. It just comes down to personal preference with the RS7 having more of a brute force feel to it whereas the Panamera is more of a scalpel. The drag racer will prefer the RS7 especially considering how tunable the 4.0 TFSI V8 is showing itself to be but the Panamera ultimately seems to be more fun to drive and more well rounded. You can't go wrong either way but MotorTrend picks the Panamera. It's difficult to disagree with their assessment.