Sorry You're right about Watts/hour.
Anyway i opted for two 4s battery connected in parallelo on this way i can get more flight time... probably 2x5000 Mah 20-30c
In HK there are a lot of batteries, i've seen 3 principal brands:
Turnigy nano-tech 5000mAh
ZIPPY 5000mAh Compact
Rhino 4900mAh...