Thermal Interface Material
Opinions vary on Bergquist Sil-Pad 400 vs mica and grease. I only use Sil-Pads as it is a very good thermal interface material (TIM) from the industry leader in TIM material. They are so close in thermal transfer both right about 0.9W/m-K that it does not matter thermally. The grease tends to get everywhere and when it comes to solderability of the transistor leads to the boards, having grease on them is not a good thing for board soldering or sockets. The Bergquist pads are not inexpensive so do not look at their use as the cheap option.
Note:
Only use Sil Pads and the 400 or better material. Bergquist is the undisputed leader in thermal management materials. They have run extensive testing on their products and have supporting data. The entire power industry relies on the Bergquist thermal materials in multi kilowatt power applications. Do not rely on good thermal performance of run of the mill pads found coming out of China on Ebay. They are not the same.
At the end of the day, do what makes you feel good but technically both are very good solutions.