How to set Steam to always choose the same launch option for a game?

I'm not sure there's a way to do that in Steam directly, but you could always create a desktop shortcut to the 64 bit KSP .exe file in the local files and launch it that way. There might be a way to do it from the "Launch Options" command line in the game's Properties, but I don't know offhand.

In your steam Library, right click KSP, press Properties, then Set Launch Options at the bottom. Type the following text into the box and press OK:

"C:\Program Files (x86)\Steam\steamapps\common\Kerbal Space Program\KSP_x64.exe" %command%

(note: if KSP is installed in a non-default location, you will need to edit the path above to match your installation. For instance, I have the game installed on my seconday hard drive under D:\Steam Library)