AS5 is SLIGHTLY (think 1-2 degree C difference) better at conducting heat, however Ceramique is an electric insulator, whereas AS5 is slightly capacitive and can cause issues if you're clumsy enough to get it somewhere other than directly on the core. Personally I use Ceramique.
Thermal conductivities from arcticsilver.com:
AS5: 350,000W/m2 °C (0.001 inch layer)
Ceramique: 200,000W/m2.°C (0.001 inch layer)
Both are better than the older AS products.