Installing an adjustable deck post support-what is the correct way to install the support?
You seem to be asking about the orientation of the threaded socket. The lower the height of the bracket, the better for stability. This favors the first option. Also, with the first option you don't have to drill for anchor bolts. I'd only use the second option if you need the height for some reason.

If you're worried about water you could fill the socket with grease or silicone caulk.