Help in Thermal PAD and glue calculation (equivalence) in shared heatsink
In a thermal system, the path of most resistance (least conductivity) will be the biggest factor in how much heat will move through the system. 

If you have plenty of cooling power, your limitation is going be getting it to the heatsinks.

1.5W/mK*0.0005m=0.00075W/K this is lower than the other pad:

8W/mK*0.0015m=0.012W/K

0.9W/mK*0.0001m=0.00009W/K which is the lowest conductivity (highest resistivity)

Regardless of the heatsink on go with the highest conductivity pad which would be the 8W/mK pad.