Which heatsink is best for a vertically standing laptop?

I'm planning on attaching one of these heatsinks to the heat pipes of my NUCXi7 computer which stands vertically on its stand.

From my horizontal testing each heatsink does about 7-10 C cooler under load, I'm just worried about the natural convection being an issue with the silver heatsink once stood up vertically.

These are both aluminum.
Silver 150x60x25
Black 150x60x18

The heatsink will be glued on semi permanently once a hole is cut into the case. I'll share final results in the next couple weeks.